Dahlias are vigorous plants and heavy feeders, requiring a steady supply of nutrients to support their size and continuous flower production. Providing the correct type and amount of fertilizer is directly linked to developing strong stems, robust root tubers, and the spectacular blooms. Understanding a dahlia’s specific feeding schedule is necessary for maximizing its performance in the garden.
Establishing the Right Fertilizer Blend
The most suitable fertilizer blend for dahlias promotes flower and tuber development rather than excessive leafy growth. This is achieved by selecting a product with low nitrogen (N) content compared to its phosphorus (P) and potassium (K) levels, such as 5-10-10 or 10-20-20. Too much nitrogen, which supports vegetative growth, results in a tall, leafy plant with few or small flowers.
Phosphorus is necessary for root and tuber growth, while potassium helps with overall plant health, stem strength, and flower quality. Fertilizers are available in two main forms: granular and liquid. Granular formulations are typically slow-release, providing consistent nutrition over weeks. Liquid feeds offer immediate nutrients to the roots, making them suitable for more frequent application.
The Seasonal Fertilization Schedule
The routine feeding schedule for dahlias begins approximately three to four weeks after planting the tubers, once the plant is established and has developed a few sets of leaves. Most growers start the feeding cycle when the plant reaches a height of about 12 inches. Starting too early is ineffective because the newly planted tuber has not yet developed feeder roots capable of absorbing external nutrients.
The frequency of application depends on the fertilizer type. If using a liquid, water-soluble fertilizer, plants in the ground benefit from feeding every three to four weeks throughout the active growing season. Container-grown dahlias require a more intensive schedule, often needing fertilization every two to three weeks, as they lose nutrients quickly through frequent watering. Applying a slow-release granular product allows for a less frequent schedule, typically requiring only one application every six to eight weeks.
Managing Mid-to-Late Season Feeding
As summer progresses and the threat of frost approaches, the feeding strategy must shift to prepare the plant for dormancy. Stopping fertilization allows the tubers to properly mature and “harden off” for winter storage. Continuing to feed late, especially with nitrogen, encourages soft, new growth vulnerable to rot and inhibits the tuber’s natural process of storing energy.
Gardeners should stop all fertilization roughly six to eight weeks before the average date of the first expected frost. This timing, often around the end of August or early September, gives the plant a necessary window to redirect energy away from foliage production. Instead, the plant consolidates carbohydrates within the tuber, resulting in a denser, well-matured tuber that is less prone to shriveling and more likely to survive winter storage.
Avoiding Common Feeding Mistakes
One frequent error is over-fertilizing with nitrogen, often by using an all-purpose blend that is too high in the first number. This imbalance causes the plant to become overly “vegetative,” producing thick, lush green stems and leaves, but few or poor-quality flower buds. The stems may become weak and spindly, unable to support the weight of the large foliage or blooms.
Another common issue is fertilizer burn, which occurs when too much concentrated fertilizer is applied at once, particularly with liquid or granular products, or placed too close to the stem. Fertilizer salts build up in the soil, drawing moisture out of the roots and causing dehydration. This results in browning edges or wilting leaves. If a plant shows signs of burn, soak the soil thoroughly and repeatedly with plain water to flush out the excess mineral salts from the root zone.