Fertilize delphiniums directly after transplanting seedlings outdoors.
More Info: Delphiniums are heavy feeders and require fertilization during the growing season.
Which Fertilizer to Use
Commercial: Any all-purpose garden fertilizer will work well, such as a 10-10-10 mix. Follow package instructions for use.
Compost: Organic compost such as manure will work well as a fertilizer for delphiniums as long as it is fully decomposed. Any compost that still consists of living matter has the potential to rob the soil of nitrogen necessary for the growth of your flowers.
How Often to Fertilize
Initial fertilization depends on the chosen planting method.
Seeds started indoors: Fertilize immediately following planting out of doors after last frost.
Seeds sown directly in the soil: Fertilize when seedlings emerge from the soil in about 4-6 weeks.
Transplants: Fertilize directly after transplanting.
Fertilize thereafter after flowering begins and six weeks later.
How to Fertilize
Spread the fertilizer around the base of your plant in small rings, close to the plant, but do not touch the plant with the fertilizer. Water the bed thoroughly after you fertilize to make the fertilizer available to the plant roots immediately. Watering thoroughly will also help to prevent the fertilizer from burning the plant.