Dahlia Pests And Diseases – Common Problems With Dahlia Plants
By: Bonnie L. Grant, Certified Urban Agriculturist
You don’t have to be a collector to appreciate the wide range of color and form found in the dahlia family. These exciting and diverse blooms are fairly easy to grow, but there are a few problems with dahlia that may limit their production and even their health. Watch for common dahlia problems and nip them in the bud quickly for continued blooms and healthy plants.
Common Dahlia Problems
No matter how experienced you are as a gardener, some issues will arise with your plants. Those amazing flowers known as dahlias are no exception. Among the most common dahlia diseases are those caused by fungi, such as powdery mildew and gray mold. Dahlia flower pests tend to reflect the sap drinking forms but can also encompass leaf eaters such as a wide array of caterpillars and larvae. Knowing what to watch for is half the battle in keeping your prized plants healthy and beautiful.
Healthy, chubby tubers are crucial to big bountiful dahlias. You will have little chance of huge bunches of flowers if your tubers are scrawny, moldy or rotting. Start with healthy tubers first.
The first signs of trouble in your plants is often just as they sprout and develop leaves. It is common to find new leaves completely chewed, lacy and barely still there. The culprits are usually caterpillars or some form of larvae. These feed on leaf tissue and make Swiss cheese of the foliage, diminishing the plant’s ability to intake solar energy. Hand picking may be the solution to beating these types of problems with dahlia.
Fungal issues such as powdery mildew are another universal complaint. Avoid overhead watering and use a horticultural fungicide.
More sinister complications can arise in your dahlias which can sometimes kill the plant permanently. These may include:
- Stem rot – Stem rot occurs when dahlias are growing in heavy, poorly drained, wet soil. Look for a white ring in the soil around the stem. The rot will creep in and kill the stem and advance down into the soil to kill the tubers.
- Mosaic virus – Mosaic virus dwarfs plants and distorts leaves. You must destroy the plant, as there is no cure.
- Botrytis – Botrytis blight is almost as bad and causes buds to rot and covers the plant with powdery gray mold. Remove any affected part and destroy it.
- Aster yellows – Leafhoppers, which are frequently found on plants, are the vector for Aster yellows, a disease where leaf-like tissue forms where you should be getting flower buds. Plants are sadly a loss.
- Viral issues – Problems with dahlias also include verticillium wilt and necrotic spot virus. Infected soil causes the former and leaves become black, brown or greenish-brown. Necrotic spot virus is caused by the feeding of thrips.
Numerous insect pests find dahlias delicious. In addition to the caterpillars and larvae, sucking insects are probably going to be the biggest issue with the most widespread offenders being:
Although these are relatively tiny pests, their habit of sucking sap from the plant can diminish its health, cause stunting and distorted plant parts and even transmit disease. Horticultural soaps and blasts of water can minimize their presence.
Several types of borer are also dahlia flower pests. Systemic fertilizers may offer some protection if applied early in the season.
Slugs, snails and cutworms produce topical damage that is visually obvious and reduces plant health and attractiveness. Hunt at night with a flashlight and use the “squish” method of pest control for cutworms. Slugs and snails may be controlled with diatomaceous earth or slug baits.
As if there weren’t enough dahlia pests and diseases, the tubers are very susceptible to freezes, which makes the plants a challenge, but a worthy one, to northern gardeners.
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Read more about Dahlia Flowers
Pests & Diseases
What Pests & Diseases do Dahlias get?
Common pests that can affect Dahlias are Thrips, Aphids, Two Spotted Mite, White Fly, Cut Worms, Mealy Bug, Snails, Powdery Mildew. and Virus.
When Controlling Pests it is very advisable to understand what your chemical will control.
It is best to use Specific Chemical for a Specific Pest.
If you use a “Kill-All Spray” you are likely to wipe out most of the predators (Goodies).
Without Predators the number of Thrip/Aphids will explode because there are no “Goodies” to assist controlling them.
Insecticide for Insects Thrips & Aphids
Miticide for Mites Red Spider (Two Spotted Mite) Broad Mite
Fungicide for Fungus Powdery Mildew,
Nematicide for Nematodes Eelworms (Nematodes)
Weedicide for Weeds Weeds
If you want to know what to use contact your Garden Centre or chemical supplier & ask for the product for the specific pest.
It is advisable to rotate your brand of chemical to reduce any buildup of resistance.
Fungus Control for Dahlias and the Garden
After a few years growing dahlias, many gardeners invariably have questions concerning fungus. This usually starts with them simply asking, “how can I prevent my tubers from rotting”, realizing in most cases the reason for the decay is caused by some form of root rot. Often they see less innocuous forms of fungus on the surface of the tubers in the form of molds (Botrytis or gray mold). Many times the presence of fungal diseases is overlooked by even the most experienced of growers. Plants with root rot disease sometimes mimic other plant problems like nutrient deficiency or heat and water stress. Plants treated with an appropriate fungicide will have larger root systems, develop faster and have improved yields. The increased root mass enables greater uptake of nutrients and water resulting in larger and better quality blooms.
The late Phil Traff attributed larger dahlia tuber clumps to his treatment with the fungicide Terraclor (pp . Dahlias of Today 198?). After more experience growing dahlias many growers begin to recognize other symptoms of fungus appearing on the plant, white patches (powdery mildew) on the green leafy growth and perhaps small dark beads (Sclerotinia) along the stems. In the case of young cuttings or seedlings they may see signs of “damping off” caused by root pathogens, especially the oomycetes commonly known as “water molds” (Pythium and Phytophthora). Occasionally the fungi Fusarium, and/or Rhizoctonia will contribute to root and stem rots Fusarium, also being the most common cause of “dry rot” in tubers. All of these diseases can be prevented with fungicides, either systemically or topically. Bear in mind, to prevent most forms of root rot internally, one must use a systemic.
However, fungicides will not be totally effective in eliminating all fungi from soil and will not cure root rot once it begins. The best cure is prevention, removing the conditions that harbor or encourage the formation of fungi. Since many fungal diseases are difficult to control once established, every effort should be directed toward prevention. Sanitation is the first important step, disinfect all pots, trays and tools with 10% bleach (sodium hypochlorite), or bromine solution. Remove dead or dying tissue from plants, clean refuse from the greenhouse and around the garden. Reduce the conditions that favor “damping-off” which frequently attacks young seedlings/plants (causing them to fall over and die):
- excessive soil moisture and excessive overhead misting.
- overfertilization and build up of insoluble salts
- low soil temperature before germination (below 68 degrees F)
- high soil temperatures after emergence (above 77 degrees F)
- overcrowded flats or seedbeds
But, no matter how careful a grower is, disease caused by soil-borne pathogens can still occur and treatment with fungicides becomes necessary.
There are two general types of fungicides, contact or protectant, and systemic. Protectant types inhibit germination of spores (small reproductive bodies) and prevent infection if applied topically to healthy tissue surface prior to infection. Once infection has occurred and fungus becomes established within the tissue, a protectant type fungicide will no longer be effective. Systemic fungicides are absorbed by the leaf tissues or root system and remain local or are translocated throughout the plant. They will not only prevent infection, but often will cure and eradicate the fungus established in the plant tissue. Because of their activity, systemic fungicides generally require one well timed application, but are more likely to cause resistance if used exclusively. Whereas, contact fungicides usually require multiple applications spaced 7-10 days apart and in general have a broad spectrum of control. When using fungicides it is important to alternate different classes (and also systemic and contact) in order to avoid populations resistant to some fungicides, a common problem, particularly with Botrytis. In general, fungi resistant to one chemical class are resistant to all other chemicals in the family. Do not rely entirely on one chemical class for control. Alternate or mix a systemic with a contact fungicide. When tank mixing two fungicides for a single spray use each at the label rate do not reduce the concentration of either. Not all are compatible with other chemicals, and some are likely to cause plant damage if used incorrectly. ALWAYS READ THE INSTRUCTIONS/LABEL BEFORE USING ANY FUNGICIDE.
Many fungicides can cause phytotoxicity or damage plants. Test a small area before treating all of the plants. It is important to keep in mind that no single fungicide will kill or inhibit the growth of all fungi. However fungicides are typically effective against “groups” of fungi and therefore a different chemical is not needed for each and every fungus. Unfortunately, since fungi start out as microscopic, (only their colonies or structures are visible), they are often difficult to identify positively without proper equipment and training. I have narrowed the fungus listed to the most common or likely to occur in a dahlia patch or around the garden (on vegetables, roses and other ornamentals). Not all fungicides listed in the table are labeled for dahlias and not all fungi listed predominately affect dahlias. Each fungicide listed is specific for certain plants and fungus pathogens. Unfortunately, many of the fungicides listed in the table for treatment of specific diseases are not readily available to the home gardener. None are highly toxic or restricted use, but they are typically packaged in large quantities (and prices) more suitable for commercial growers. This is regrettable, because many of the fungicides commonly available at local garden shops do not treat the myriad of fungal diseases one is likely to encounter. (Most of these fungicides can be purchased through horticultural or agricultural supply companies, e.g. E.C. Geiger, Brighton By-Products, D&L Grower Supplies, BFG -Burton Flower & Garden, etc). Check labels and ingredients carefully, many of the active ingredients are labeled under a variety of trade names. Also, check the concentration of active ingredients, which can vary in different formulations (liquid, powder, granules, etc). Finally, be sure to only use a fungicide on plants listed on it’s label it is illegal otherwise.
There are many classes of fungicides ranging from the older controversial EDBCs to the newer biological controls. Each class varies in its mode of action, spectrum, physical properties in the plant, and its ability to withstand resistance. As new fungicides are developed older ones are replaced. We can expect to see a new class of fungicides, called the “Strobilurin” group, becoming increasing available for home use. These highly active fungicides are considered “fast track” or “green” compounds by the EPA. In general, they are very safe to the applicator and to the environment. They are used at relatively low rates (0.2-0.25 lb. of active ingredient per acre) and can control all four groups of fungi. Because they degrade fairly rapidly they don’t contaminate soil or ground water or harm wildlife. The Strobilurin compounds have a new and novel mode of action for control of fungi. They work by inhibiting the mitochondrial electron transport system in plant pathogens, but not in other living entities. Thus they are both effective and selective. Chemicals in this group include kresoxym-methyl, azoxystrobin, trifloxystrobin, and others.
Powdery Mildew (Erysiphe cichoracearum): – is characterized by a fine white (or light gray) talcum-like appearance on leaf surfaces (tops or bottoms) and along stems. Powdery mildew seldom kills the plant, but its infection can severely limit the plant’s health and growth. Infection peaks under warm days with low humidity, and cool nights with high humidity conditions (and is in fact inhibited by moisture on the leaves).
Rhizoctonia (solani)(brown patch): – is a soil borne fungus known to cause root rots, stem rots and “damping-off”. It is favored by warm, moderately moist soil conditions. Once the fungus begins to grow in the plant, the infected areas decay rapidly. Under high humidity the fungus will quickly spread to neighboring leaves. The leaf spots produced by Rhizoctonia are very peculiar to this type of fungus. They may be as large as 1/4 inch and are oval or peanut-shaped. The most distinct feature of the spots is the clearly defined, light-colored outline. Inside this clearly defined outline will be an area of mottled discoloration marked by “zones,” i.e., spots within spots. This discolored area inside the outline will range from light green to yellow, especially in the early stages of the fungus. As the fungus progresses, however, the spots will begin to turn more brown in color. Symptoms are wilt during midday and stem rot at the soil line with brown to reddish lesions. Soil will often cling to the cankered areas of the plant when removed from the soil because of the coarse brown mycelium (body of the fungus). Rhizoctonia also favors stress conditions including soil compaction and excessive drying and rewetting of the soil medium causing the roots to crack and decay.
Pythium: – is a common fungus found in soil, sand and water that is the major cause of root rots. It is one of the fungi in the oomycete class often referred to as “water molds”. It is easily introduced in soilless mixtures with dirty tools, pots or flats. Almost all plants are susceptible to Pythium root rot, which can rot the base of unrooted cuttings. In general, Pythium is restricted to young or succulent plants. Conditions that favor root rot pathogens are high soil moisture, high soluble salts and poor aeration. Symptoms of infection include stunted plants that wilt at midday and recover at night brown tissue on the outer portion of the root easily pulls off leaving a bare strand of vascular tissue exposed. Rot will often proceed up the stem eventually causing the plant to yellow and die. Pythium’s white cobwebby growth (mycelium) can often be seen on dew-covered grass in early morning.
Phytophthora (spp)– rot is related to Pythium and as one of the fungi in the oomycete class, has similar symptoms and treatments as Pythium. A soil-borne fungus, it is a common contributor to seed rot and pre-emergent and post-emergent “damping-off”. This disease was the cause of the Irish potato famine (1843-47). The fungus can be airborne, waterborne and seed-borne. The disease can first appear as water-soaked areas on the leaves of plants, with a white mold on the underside, areas around the lesions on the stem shrivel and pinch the seedling off at the base.
Downy Mildew: – (not related to powdery mildew) is in the same class of fungi as Pythium and Phytophthora (oomyctes) and is considered a “water mold”. The first symptom is green to yellow spots on the upper surface of the leaf. When the leaf if overturned a downy white to gray growth (tuft) may be seen opposite the upper area of sporulation. Confirmation of downy mildew infection must be made with a microscope. Downy mildew infection favors high humidity, long duration of leaf wetness and cool weather (60-74 degrees F). Damage from downy mildew rarely results in the death of the plant.
Fusarium– is predominated by two forms, Fusarium solani a fungus often referred to as “sudden death syndrome” and Fusarium spp. which causes “dry rot”. F. solani causes yellow interveinal blotches, which eventually become necrotic, leaving green tissue along the leaf veins. Leafs often drop leaving petioles attaches to the stem. It is a weak pathogen requiring some kind of predisposing factor that stresses or injures the plant before it can become established, thus it is often a secondary invader. In tubers it causes brown rings and most importantly results in tuber rot during storage. Fusarium spp. produces light to dark lesions on tubers. The rotted tissue is relatively firm, but as the lesion matures, the tissues become dry and punky, and may contain areas that are yellow, pink or orange.
Black Spot (Diplocarpon rosae) – commonly appears on roses as black purplish spots and areas of yellowing (chlorosis) on upper surfaces of the leaf and on stems and branches. At the beginning of the growing season, blackspot will start to develop on the lower leaves and will move upward through the plant as the season progresses. Infected foliage eventually turns yellow and falls off the plant. Heavy infections can seriously defoliate a plant. Blackspot development is favored by warm (75°F), wet weather. Since the spores must have free water in order to germinate, avoid wetting the plants when irrigating (or do so early in the day, allowing plants sufficient time to dry).
Anthracnose – The symptoms of Anthracnose vary somewhat on different hosts, but typically the spots start on the leaves as water soaked areas and expand into brown spots which are roughly circular, reaching about 1/8 to 1/2 inch in diameter. The centers of these spots may drop out, producing a “shot hole” effect. Small, growing leaves may be distorted and severe spotting may cause entire leaves to blight. The fungal spores depend upon water for spread and infection warm and humid rainy weather at frequent intervals is necessary for disease development. The spotted or striped cucumber beetle is another common vector that can carry the spores from plant to plant.
Sclerotinia – two species of Sclerotinia can cause disease on dahlias. Sclerotinia minor only infects stems or leaves in close contact with the soil. Once infection takes place, water-soaked brown necrotic areas develop. Plants eventually wilt and collapse. Upon examination of diseased plants one finds cottony fungal growth and numerous small, black, hard resting bodies called sclerotia (large reproductive structures) formed on the outside and inside of the stems. Sclerotinia sclerotiorum (white mold) can also infect lower leaves and stems, causing similar symptoms as S. minor, but S sclerotiorum also has an aerial spore that can infect upper leaves and flowers. S sclerotiorum forms larger sclerotia than those of S. minor. S sclerotiorum prefers cool and moist conditions and the aerial spores usually only infect injured leaves and flowers. The sclerotia may survive for years in infected soil.
Thielaviopsis (basicola)(black root rot) – is a soil inhabiting fungus that causes root rot and branch dieback on a number of woody and herbaceous plants. Soil temperatures between 55 and 65 degrees F favor the growth of Thielaviopsis, particularly if the soil is wet. Symptoms include stunting of foliage and root systems. Most dominant are blackened areas on roots. Often there is yellowing of leaves between the veins or along the margins (frequently mistaken for virus). Lower stems have sunken lesions.
Botrytis (cinerea) – commonly called Botrytis blight or gray mold first appears as a fuzzy white growth on the plant, but quickly darkens to a gray color. Spores multiply rapidly and can thrive on many different sources of nutrients. Botrytis is a weak pathogen that must have a food source before it can invade the plant, such as nutrients leaking from wounded plant parts or dying tissues such as old flower petals or leaves. Botrytis thrives in conditions of high humidity, poor air circulation and cool temperature. As with many other fungi, Botrytis has been known to develop resistance to many fungicides, particularly systemic fungicides when used exclusively over a period of time.
Fungus Gnats – while not a fungus, these small (1/16 in) insects feed off fungus and decaying plant material. They are slender, mosquito-like insects with long legs and many-segmented thread-like antennae. The gnats are often seen swarming around plants when fungus is present. While little threat to mature plants, they can damage young seedlings by feeding on roots and tunneling into stems, causing plant death. In addition, the larvae and adult fly are capable of spreading fungal diseases including Pythium, Thielaviopsis, and Botrytis. They can be treated with a general purpose insecticide such as Orthene, Diazinon, Talstar or Gnatrol.
What Are Dahlias?
Close-up of Frank Holmes Purple Pom Pom Dahlia.
Dahlias originate from Mexico and would you believe that they were once eaten just like a potato plant. Can you imagine growing gorgeous dahlias to eat the tubers? Crazy, huh? In my wildest dreams I can’t imagine growing dahlias just to eat their tubers.
What Are Dahlias According to Wikipedia
According to Wikipedia, a “dahlia is a genus of bushy, tuberous, herbaceous perennial plants native to Mexico and Central America.” Dahlias are a member of the Asteraceae family. They are closely related to sunflowers, daisies, chrysanthemums, and zinnias. There are hundreds of different varieties of dahlias. All dahlia plants produce single stem flowers ranging from 2 inches in diameter to a foot wide. Most dahlias do not produce any scent and they attract pollinators to the garden through their bright colors.
|Lush Foliage Sparse Bloom||Over-fertilizing|
|Plants Don't Bloom in Summer||Weather Too Hot|
|Buds Discolored Tunnels in Stems Plants Wilted||Borer|
|Leaves Discolored Distorted Growth Stunted||Leafhopper|
|Leaves Curled And Distorted||Aphid|
|Leaves Discolored And Deformed Webbing Present||Mites|
|Leaves And Flowers Discolored And Withered||Thrips|
|Leaves Covered With White Powder||Powdery Mildew|
|Leaves Turn Yellow and Wilt||Stem Rot|
Over fertilizing Causes Lush Foliage, Sparse Bloom
If plants receive too much nitrogen fertilizer, they often stop blooming and put all their energy into leaf growth. Leach excess nutrients from the soil by watering thoroughly several times. Use only a slow release type of nitrogen fertilizer.
Weather Too Hot Plants Don't Bloom In Heat
Sometimes during the dog days of August, dahlias stop blooming or bloom sparsely. Be sure they are well watered and wait until cooler weather, when they will spring back.
Buds Discolored Tunnels in Stems Plants Wilted - Borer
European corn borer and stalk borer sometimes attack dahlias. The stalk borer is a striped worm is more likely to appear in early summer, whereas the corn borer arrives later in the summer. These nuisances feed on bud tips, flowers and leaves and then, left unchecked, burrow down inside the dahlia stem. This causes the plants to wilt. To treat borer attacks, spray BT (Bacillus thuringiensis) on the dahlia foliage when the worms are feeding. They will ingest the bacteria, sicken and die within a matter of days. Repeat applications of BT if rain washes it away.
For more information see the file Controlling Borers
Leaves Discolored, Distorted Growth Stunted - Leafhopper
The potato leafhopper is a small, slender, pale green insect about 1/8 inch long. These pests cause leaf discoloration, which appears first along one margin and spreads toward the mid-vein. The affected area is at first pale yellowish, later turning brown and brittle. Infested dahlia plants are often stunted. Apply a preventive spray of insecticidal soap and seaweed extract to vulnerable plants during the first month of growth. Later, spray any leafhoppers with insecticidal soap laced with rubbing alcohol.
For more information see the file on Controlling Leafhoppers
Leaves And Flowers Discolored And Withered - Thrips
Adult thrips are tiny, slender insects, 1/25 inch long, variously colored pale yellow, black or brown, with four long, narrow wings. Their larvae are usually wingless. Thrips infest flowers by rasping the surface and feeding on the exuding juice, which causes petals to turn whitish and wither. Set out yellow sticky traps about 4 weeks after last frost as early warning devices. As soon as you spot thrips on the traps, spray vulnerable plants with insecticidal soap every 3 days for 2 weeks.
For more information see the file on Controlling Aphids
Leaves Covered With White Powder - Powdery Mildew
Fungi that live on the surface cells of the plant cause powdery mildews. Infested dahlia leaves are covered with a white or ash-gray powdery mold. Badly infected leaves become discolored and distorted, then drop off. Powdery mildews thrive in both very humid and very dry weather. Spray affected plants thoroughly with wettable sulfur once or twice at weekly intervals starting as soon as the whitish coating of the fungus is visible. Allow ample spacing between plants and collect and discard all aboveground refuse in the fall.
For more information see the file on Controlling Fungal Disease
Leaves Turn Yellow and Wilt- Stem Rot
Soil-dwelling fungi cause root and stem rot of dahlias. Typically, plant stems are attacked at or near the soil level. Foliage turns yellow, wilts, and dies. Usually root systems rot, causing plants to topple over. Remove and discard infected plants, or cut away affected plant parts with a clean, sharp knife or razor blade. Disinfect tools after use in a bucket of water and household bleach. Keep the yard clear of old plant debris and keep mulch away from stem bases. For long-term prevention, lighten heavy soil with a mixture of perlite, vermiculite or peat moss and provide good drainage. Avoid over watering and space plants further apart to prevent crowding.
Do you have a gardening question? Ask Nancy
Dahlia Insects and Pest Control
Slugs, and Snails
After you have completed your planting, there is notihing to do but wait until the dahlias grow. The most common pests are snails, and slugs. They will eat your tubers underground, as well as the new growth just above the soil. We use Sluggo, or the slug pellets that can be purchased at most home and garden stores.
Earwigs, Cucumber Beetles, Spider Mites
During the growing months, earwigs, cucumber beetles, spidermites, and other garden pests will thrive on the flowers, and foliage. We spray our dahlias with Orthenex a few times a year, since we do sell the flowers.
How to Grow Dahlias in the Summer
Most dahlias will be blooming on average of 90 days after planting. The earliest blooming dahlias will be most of the Low Growing Dwarf dahlias and early blooming varieties, which will bloom approximately 75 days after planting. While the larger blooms (8" bloom size and larger) will take approximately 120 days from planting to bloom time. The great thing with dahlias though regardless of when they start to bloom they will bloom all the way through frost. However, this bloom time greatly depends on the growing environment including weather, water, and fertilizer given.
Watering is the #1 problem we see when dahlias are not growing and blooming properly. They do not need to be watered at planting time in most areas but need lots of deep (8-10") watering to reach the roots once they sprout above ground level. The exception will be in very warm, dry areas, where light watering at planting is needed (about once a week) until the dahlias have emerged. Most areas have an adequate amount of rainwater to get dahlias off on the right foot. Once the dahlias are above the ground and established they will require deep watering 3-4 times per week for 60+ minutes by sprinkler or soaker hose, and even longer in the heat of the summer. Deep watering means the water will reach down eight or more inches to the planted tuber's roots. Hand watering is not enough nor is it a method we recommend! Watering deeply three times a week is better than watering lightly 7 times a week as the water will not reach the roots leaving you with a green plant and little to no blooms. Please keep in mind your climate will determine the amount you will need to water. Adequate deep watering is required for proper growth and blooming. Most rain showers will not penetrate the soil deep enough for adequate watering.
Use Low Nitrogen Fertilizer
We recommend a low nitrogen fertilizer, usually referred to as a bloom food. For dahlias there are two options, one is processed/bagged steer or cow manure, the second is commercial fertilizer or a combination of both. We recommend using both. Dahlias require low nitrogen fertilizer. We recommend high percentage potassium and phosphorus fertilizers. A good rule of thumb is looking for a fertilizer where the first component number is 1/2 of the other two numbers. Usually, a bloom food or vegetable type fertilizer will have these similar numbers. The first application should be 30 days after planting and repeated every 3-4 weeks throughout the season, about 1 time per month is fabulous. If growing in containers you will need to fertilize every 2 weeks, as the fertilizer is washed through the soil quicker. Please remember that during extreme heat or any event causing stress to your plants you will need to fertilize more often, as they are using up the fertilizer quicker.
Can be applied anytime prior to planting, during planting, or during the growing season. This will amend and lighten your soil and also fertilize. If you choose to steer manure it must be processed/ bagged manure. The only recommended manure is a cow or steer, all others are too rich and will burn the tubers. If you want to add fresh steer manure, add it in the fall, this will allow time for it to break down prior to spring planting. If your plants aren't thriving they are needing more fertilizer and/or more water.
All commercial fertilizers contain a 3 number component series. The first number indicates the amount of Nitrogen, the second number indicates Phosphorus, and the third number is Potassium (Potash). We recommend fertilizers low in nitrogen and high in potassium and phosphorus. We like to see the first component half, or less than half, of the second two components. Examples would be 2-4-4, 3-9-4, 4-10-7, 5-10-10, 2-15-15, 8-16-16, 6-30-30. Any fertilizer close to these numbers will work great! The second two components do not have to be exact matches, just as close as you can get. Look for a bloom food that mentions that it is a bloom booster or promotes blooming. We recommend staying away from Miracle-Gro products as most are high in nitrogen. The first application of fertilizer for your dahlias should be about 30 after planting and repeated every 3-4 weeks throughout the season. Follow the manufacturer’s directions on the application amount. One of the biggest mistakes with dahlias is overfeeding them with high nitrogen fertilizer.
Avoid all high nitrogen fertilizers
One of the biggest mistakes made with dahlias is overfeeding them with “High Nitrogen” fertilizers. Avoid High Nitrogen compost and High Nitrogen water-soluble types (ie: Miracle-Gro - or anything with 16-16-16, 18-18-18, 15-0 0, 20-5-5, etc - Triple digits matching or the first number higher than the other two numbers is too much Nitrogen) as they promote weak stems, huge green plans, small blooms, blown/popped centered blooms, or no blooms, and tubers that rot or shrivel in storage. Only use Zero Nitrogen late in the season (ex: 0-10-10 Alaska Morbloom). A fertilizer with zero nitrogen (the first number in the 3 digits listed on a fertilizer), such as 0-10-10 should only be used late in the blooming season to increase bloom.
Dahlias will hibernate if they are not receiving proper water or fertilizer. If your dahlias are green plants lacking blooms, then most likely they are waiting for more water, food, and/or direct sunlight. Dahlias require deep watering 3-4 times a week by a soaker hose or sprinkler for 60+ minutes each session when planted in the ground, as the water must reach the roots that are 8-10” deep, or watered at least 1-2 times daily if planted in pots. Hand watering with a hose or watering can is not sufficient for dahlias and will not allow the water to get deep enough to their roots. Dahlias also require low nitrogen fertilizer which should be applied about every 3-4 weeks for in the ground plants and every 2 weeks for dahlias growing in pots. They also need 6-8 hours of direct sunlight each day to thrive. If your dahlias are struggling in the summer, please give them 1 dose of high nitrogen fertilizer and ½ cup of Epsom salts to help them take off. Two weeks after the one application of high nitrogen fertilizer, please switch back to low nitrogen fertilizer and apply it every 3-4 weeks. Please increase your water if you are not giving them the suggested amount per our recommendations. They should take off in 10-14 days once our recommendations are followed.
We recommend staking all dahlias that will reach 3 feet or taller. Any staking product will work, please check your garden center - ie: tomato cages, metal rods, or bamboo stakes. It is best to put your stake in at planting time, however, stakes can be added anytime during the season. Please visit our garden shop for staking supplies.
To promote shorter, bushier plants with better stem length for cutting, pinch or cut the center shoot just above the third set of leaves or when the plant height is about 18-20" tall. This is just a one-time pinch. Please see the diagram or video on our website dahlias.com for additional information. If at any point your dahlias are too tall, they can be trimmed back. See videos at dahlias.com