Fertilizer for Dahlias: NPK Guide and Application Rates
Dahlias are moderate feeders that produce their best blooms with a low-nitrogen, high-phosphorus and potassium fertilizer once buds begin to form. Oregon State University Extension recommends avoiding heavy nitrogen feeding, which promotes lush foliage at the expense of flower production. Wait until plants are 12 inches tall before the first application, and stop fertilizing by late August to encourage tuber storage for winter.
Quick Reference
What NPK Ratio is Best for Dahlias?
The recommended NPK ratio for dahlias is 5-10-10. The phosphorus-heavy 5-10-10 ratio supports strong root systems and flower/fruit production in dahlias.
Dahlias NPK needs shift at each growth stage. Balanced formula during early sprout growth from tuber during the Seedling phase, then low nitrogen with high phosphorus and potassium for abundant blooms once flowering begins.
NPK by Growth Stage for Dahlias
| Growth Stage | NPK Ratio | Notes |
|---|---|---|
| Seedling | 10N 10P 10K N 10% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | Balanced formula during early sprout growth from tuber |
| Vegetative | 10N 10P 10K N 10% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | Balanced feeding until flower buds appear |
| Flowering | 5N 10P 10K N 5% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | Low nitrogen with high phosphorus and potassium for abundant blooms |
| Fruiting | 0N 10P 10K N 0% Nitrogen: leaf and stem growth P 10% Phosphorus: roots and flowers K 10% Potassium: health and disease resistance Learn more about NPK | No nitrogen in late season to promote tuber storage |
When Should You Fertilize Dahlias?
In zone 3, start fertilizing dahlias in June. In zone 10, begin as early as March. Apply every 3 weeks through the growing season.
Which Zones Can Grow Dahlias?
Dahlias grows in USDA zones 3 through 10, spanning a 8-zone range.
Fertilization Start by Zone
| Zone | Start Month |
|---|---|
| Zone 3 | June |
| Zone 4 | May |
| Zone 5 | May |
| Zone 6 | May |
| Zone 7 | April |
| Zone 8 | April |
| Zone 9 | March |
| Zone 10 | March |
What Are the Best Fertilizer Products for Dahlias?
Look for products with an NPK close to 5-10-10. Espoma Flower-tone is the most popular choice among home gardeners for dahlias.
- Espoma Flower-tone
- Jack's Classic Blossom Booster
- 5-10-10
Prices vary by retailer and region. Check current pricing before purchasing.
Container and Raised Bed Adjustments for Dahlias
Grow in 12+ gallon containers; feed every 2 weeks with liquid bloom fertilizer
Common Fertilizing Mistakes with Dahlias
- Applying high-nitrogen fertilizer throughout the season, producing enormous plants with few flowers.
- Fertilizing tubers at planting time before roots have developed, causing tuber rot.
- Continuing to fertilize into fall, which prevents proper tuber hardening for winter storage.
- Watering overhead after fertilizing, which can cause foliar burn on dahlia leaves.
Tips for Fertilizing Dahlias
- Wait until plants are 12 inches tall before first fertilizer application to avoid burning young roots.
- Switch from balanced to bloom-boosting fertilizer (5-10-10) once flower buds begin forming.
- Stop all fertilization by late August to allow tubers to store energy for winter dormancy.
- Disbud side shoots for fewer but larger dinner-plate blooms, or leave them for more prolific garden display.
Organic Fertilizer Alternatives for Dahlias
Sources
Source:Oregon State University Extension - Dahlias in OregonLast updated: