Fertilizing Annual and Perennial Flowers in the Garden

Fertilizing Ornamental Flowers

Brian Jervis: Master Gardener

Tuesday, May 30, 2017

Q: How often should I fertilize my flowers and what type of fertilizer should I use? Sue, Tulsa

A: How often to fertilize and what fertilizer to use, of course, depends on the type of soil and the variety of ornamental in your garden.

It is always best to start with a soil test. A test will determine the nutrient content of your soil and serve as a guideline for deciding which fertilizer to use. Instructions for collecting soil samples are in the soil section of the Master Gardeners website, tulsamastergardeners.org.

Generally, most annual plants will benefit from fertilizer during the growing season. Perennials, if mulched regularly, will usually get enough nutrients from the mulch.

As a rule, extra nitrogen fertilizer will be needed by most plants. Nitrogen is used in large amounts by plants and what is not absorbed is often washed deep into the soil.

The other nutrients — phosphorus and potassium — do not migrate in the soil, remaining where applied if not used by plants. Their behavior is such that they leave a residual if a landscape has previously been fertilized. Applying more of these two, especially phosphorus, can be harmful to the environment. A soil test will sort this out.

When deciding on what fertilizer to use, there is a choice between conventional and organic fertilizers. Although the major nutrients in each are exactly the same — plants can’t tell the difference — there are some pros and cons of each type.

With commercial fertilizer, you are aware of the exact amount of each nutrient, it is easier to apply and, overall, cheaper.

Organics are great in that they not only add nutrients but also help to make sandy and clay soils more plant friendly by improving structure. Another advantage of organics is that they also have minor nutrients and beneficial soil organisms not found in most commercial preparations. One of the downsides to organics is they have a lower concentration of the major nutrients and need to be used in larger amounts. This often means that it requires more effort and may be more expensive.

Some of the organic fertilizers with the highest concentration of nitrogen to consider for use are cottonseed meal, blood meal, bat guano, fish meal, fish powder, fish emulsion, soybean meal and milorganite.

A general take home message for fertilizing flowers might be this: Fertilize annuals and perennials at the time of planting with slow-released commercial or high-nitrogen organic fertilizer. In the growing season, perennials can be fertilized once again. Most annuals will benefit from fertilizing with a liquid nitrogen fertilizer or an organic preparation every 2-3 weeks during the summer and while blooming.

The last thing one should do is to develop the “more is better” mindset and use too much fertilizer. Most experts think that there is more harm to plants by over-fertilizing, than not fertilizing at all.

Garden tips

Clean out water garden and prepare for the summer season. Divide and re-pot water garden plants. Begin feeding fish when water temperatures are higher than 50 degrees.

Plant warm-season vegetable crops such as watermelon, cantaloupe, cucumber, eggplant, okra, sweet potatoes, etc., now.

Fruit trees, especially apples and peaches, must be thinned out for best production. Prune apples 4-6 inches apart and peaches 6-8 inches. This will ensure larger fruit and less damage to limbs. If not thinned, the tree's resources will be used to such an extent that next year’s crop will suffer.

Late May is the best time to control borers in the orchard. Contact OSU Tulsa Master Gardeners for fruit tree spray recommendations.