Quick Answer
Dahlias should be fertilized every 2-4 weeks during the active growing season for best results. Use a balanced fertilizer formulated for flowers by following label directions. Start fertilizing when the plants are 1-2 feet tall and stop 6-8 weeks before your first expected fall frost date. Over-fertilizing can cause excess foliage growth at the expense of flowers.
When to Start Fertilizing Dahlias
You can begin fertilizing dahlias once they reach 1-2 feet tall, which is usually 4-6 weeks after planting tubers or transplanting young plants. Dahlias are heavy feeders and benefit from regular fertilizer applications during their active growth period. Wait until the plants are established and putting on vigorous growth before introducing fertilizer. Starting too early can burn young plants.
Fertilizer Types
Use a balanced flower fertilizer with an NPK ratio such as 10-10-10 or 15-15-15. An all-purpose garden fertilizer can also be used at half strength. Avoid high nitrogen lawn fertilizers which can promote excess foliage over flowers. Organic fertilizers such as compost, fish emulsion, worm castings, or manure can also provide nutrients for dahlias and help improve soil structure over time.
Fertilizing Frequency
Plan to fertilize dahlias every 2-4 weeks during the growing season for best performance. More frequent light applications are better than heavy applications less often. Spread fertilizer around the base of the plants, following label directions for product and plant size. Water in after application to avoid burning leaves.
Application Method
Fertilizer can be applied in several ways:
- Spread granular fertilizer on the soil around dahlias and water in thoroughly. Follow label directions for amount.
- Mix liquid fertilizer into watering can at directed dilution rate and apply to soil around plants.
- Lay a ring of fertilizer 6 inches away from plant; the nutrients will reach roots when watered.
Avoid placing fertilizer directly on plant leaves which can burn. Always water after applying to move nutrients into the root zone.
Signs of Fertilizer Needs
Watch for signals that your dahlias need additional fertilizer:
- Slowed growth
- Loss of lower leaves
- Pale green or yellowed leaves
- Small flowers
- Less than normal blooms
These are indications the soil lacks adequate nutrients for vigorous plants and abundant flowers. Apply a balanced fertilizer soon after noticing these deficiency signs.
Stopping Fertilizer in Fall
Discontinue fertilizer applications about 6-8 weeks before your average first fall frost date. This allows plants to harden off and prepare for dormancy. Late season fertilizing can stimulate succulent new growth that is susceptible to cold damage. Dahlias form next year’s tubers in late summer and autumn, so some key nutrients are still needed until the end of the season.
Over-fertilization
It is possible to over-fertilize dahlias, which can cause issues:
- Excess foliage growth instead of flowers
- Floppiness in the stems
- Delayed or reduced blooms
- Increased disease and insect susceptibility
To avoid over-fertilizing, do not exceed label rates and reduce frequency if plants show signs of excess growth and low flowers. Excess nitrogen is usually the culprit, so switch to a low or no nitrogen formula.
Key Nutrients for Dahlias
Dahlias need a constant supply of essential macro and micronutrients for optimal growth and flowering:
Macronutrients
- Nitrogen (N) – Promotes leaf/stem growth; needed for photosynthesis.
- Phosphorus (P) – Important for root development and flowering.
- Potassium (K) – Necessary for flowers and tuber formation.
- Calcium (Ca) – Strengthens cell walls; improves quality.
- Magnesium (Mg) – Key for photosynthesis and energy production.
- Sulfur (S) – Helps form nutrients and amino acids.
Micronutrients
- Iron (Fe) – Essential for photosynthesis and enzyme functions.
- Manganese (Mn) – Needed for nutrient uptake and metabolism.
- Zinc (Zn) – Important for growth regulators and metabolism.
- Boron (B) – Helps with calcium uptake and cell wall strength.
- Molybdenum (Mo) – Converts nitrates into usable forms for plants.
- Copper (Cu) – Key element in enzymes and vitamin production.
A balanced fertilizer contains all the necessary macro and micronutrients. Quality compost also provides a diversity of nutrients.
Soil pH for Dahlias
Test soil pH annually and adjust if needed to maintain the ideal range. Dahlias grow best with a slightly acidic soil pH between 6.0-7.0. Use lime to raise pH or sulfur to lower pH according to soil test recommendations. Proper pH helps dahlias access nutrients.
Using Compost
In addition to providing nutrients, compost improves soil texture, increases water holding capacity, and enhances overall soil health. Incorporate 1-2 inches of good quality compost into dahlia beds each spring before planting. Topdress established plants with 1/2 inch of compost or finely screened leaf mold in early summer to provide organic matter and nutrients.
Preventing Fertilizer Burn
While dahlias need fertilizer for peak growth, it is possible to over-fertilize and burn plants. To avoid this:
- Follow label instructions carefully.
- Avoid overapplying nitrogen which causes excessive leaf growth.
- Never place dry fertilizer directly on plant leaves.
- Water thoroughly after each application to soak fertilizer into soil.
- Watch for signs of fertilizer burn: brown leaf margins or yellowing.
Reduce frequency and concentration if fertilizer burn occurs. Leach excess fertilizer from soil with gentle, steady watering over time.
Using Organic vs Chemical Fertilizers
Organic fertilizers like compost, manure, fish emulsion, etc. provide nutrients through slow, steady decomposition. Chemical fertilizers are immediately soluble in water for rapid uptake. A balanced feeding program uses both:
- Incorporate compost each spring for sustained release nutrition plus soil conditioning.
- Supplement with periodic liquid organic fertilizers like fish emulsion during summer.
- Use chemical fertilizers for immediate, measured nutrients as needed to maintain growth.
Choose non-urea based chemical fertilizers to avoid volatilization loss in hot weather. Always follow label instructions to avoid burning.
Foliar Feeding
Foliar sprays applied directly on dahlia leaves provide micronutrients quickly but are not a substitute for soil-applied fertilizer. Use foliar feeds containing seaweed extract, fish emulsion, compost tea, or commercial flower formulations to give plants a nutrient boost:
- Early in the season to support rapid growth
- After pruning to refuel plants
- During bud set to maximize blooms
- If deficiency symptoms appear
Spray early in the day to allow leaves time to dry and avoid sun scorch. Foliar feeding is especially helpful if soil nutrients are lacking and immediate correction is needed.
Testing Soil Nutrient Levels
A soil test every 2-3 years reveals pH and nutrient levels in your dahlia beds. Samples can be analyzed through local extension services, universities, or private labs. Soil testing determines:
- Soil pH
- Macronutrient levels – N, P, K
- Micronutrient levels – iron, manganese etc.
- Recommendations for amendments to correct deficiencies and pH
This information allows you to tailor fertilizer applications to the specific needs of your soil and avoid guesswork.
Special Considerations for Container Plants
Potted dahlias grown on patios or in pots require more frequent feeding than in-ground plants. Consider these tips for fertilizing container dahlias:
- Use a slow release pellet fertilizer at planting time to provide steady nutrients.
- Supplement with liquid fertilizer every 2 weeks according to label directions.
- Increase frequency for fast growing, multi-head varieties.
- Check soil moisture frequently and water as needed between feedings.
- If using compost, repot plants into fresh mix annually.
The limited soil volume in containers cannot hold nutrients indefinitely. Monitor plants closely and be prepared to fertilize more often to support growth and flowering.
Common Signs of Nutrient Deficiencies
Monitor dahlia leaves and growth regularly for any signs of insufficient nutrients:
| Problem | Potential Deficiency |
|---|---|
| Weak, stunted growth | Nitrogen |
| Chlorosis – yellowing leaves | Nitrogen, sulfur, iron |
| Brown spots and margins on leaves | Potassium |
| Purple leaf coloring | Phosphorus |
| Yellowing leaf veins | Iron, manganese |
| Tiny new leaves | Calcium |
| Poor flowering and tuber growth | Phosphorus, potassium |
Identify the specific nutrient that may be lacking based on symptoms and correct with foliar sprays or soil applications of that element.
Conclusion
Fertilizing dahlias regularly is important to fuel profuse growth and maximize flowers from midsummer until fall. Use a balanced flower fertilizer every 2-4 weeks, and supplement with organic matter like compost. Monitor plants closely for signs of nutrient deficiencies or fertilizer burn and adjust applications accordingly. With proper feeding, your dahlia garden will produce an abundance of spectacular blooms.
