KC Gardens

To grow robust annual flowers, fertilize early and often

Fertilize your petunias and other annuals when you plant them and again each month.
Fertilize your petunias and other annuals when you plant them and again each month.

Brightly colored annual flowers are popping up around the metro area. Annuals are an easy way to provide color with little care, but you’ll get the most from your plants if you give them a little attention. Here is what you need to know to achieve maximum results.

Know how buds form

Annuals have a limited time to flower before frost puts an end to the color. The goal is to get as many flowers as possible in that short period of time. Breeders have been working hard to simplify the process, but the plants still need assistance.

Blooms form at the end of a healthy, vigorous vegetative shoot. The biggest, most robust plants are those that produce a lot of new green growth in which the flowers form. So your goal is to produce more of that green growth, which means feeding your plants consistently and often.

Many times annuals underperform because they are not fed enough to push the growth. The secret to fertilizing annuals is to keep it up all season long.

Start at planting. This application helps the plants get established. To keep them growing strong, feed them monthly through September.

Annuals Rudbeckia Horizontal
Annuals such as Rudbeckia produce abundant flowers when the plant has robust green growth. Adele Wilcoxen

More nitrogen, less phosphorus

Marketers of plant fertilizers have not always based their information on sound horticulture research. Many flower fertilizers have higher amounts of phosphorus and lower levels of nitrogen. With names like “bloom buster,” these products miss the research-based point that flowers are produced from new green growth.

While phosphorus is important, it is associated with root development, not flower bud formation. Look for the three numbers on the product label. If the middle (second) number of the three is the highest, then put it back on the shelf. The first number in the nitrogen, phosphorus and potassium ratio is the most important: Nitrogen is responsible for green shoot development.

Fertilizers labeled for flowers are not always properly formulated or easy to use, and they may not provide the best results.

How to fertilize

Annuals Marigolds Vertical
Annuals such as marigolds flourish if they are fed steadily throughout the summer. Adele Wilcoxen

The problem for many gardeners is fertilization involves math and soil chemistry. Most of us just want simple, clear directions. If you have not had a proper soil test, which provides all the information you need, here is the recipe for flower fertilization for dummies.

Once a month evenly spread a fertilizer such as 10-10-10 or something similar at the rate of 1 pound, about 2 cups per 100 square feet of bed area. Estimate your square footage by multiplying length times width. For small plantings of five or six annuals, sprinkle 2 to 3 tablespoons of fertilizer over the area. Be sure to water it in after the application. This recipe is easy to follow and inexpensive.

That’s it, all you need to know for a colorful summer of flowering annuals.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to garden.help@jocogov.org.