When and how often should you apply plant fertilizer: Tips, timing, more in Dan Gill's mailbag (2024)

QUESTION: I'm trying to decide when to fertilize the many different kinds of plants in my landscape. I don't know when to start fertilizing or when I should stop. What is the best fertilizer to use and how often should it be applied? -- Sam DuPont

ANSWER: You are not alone. I get these questions all the time. First, keep in mind that nothing will die in your landscape if you don't fertilize. Fertilization is not done to keep plants alive, but to make sure they grow their best. You do not need a ridged schedule that has to be followed precisely. It's not that complicated.

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Generally speaking, the best time to fertilize landscape plants is around the time they begin to grow actively. The worst time to fertilize plants is at the end of their growing season. Trees, for example, begin to wake up and grow in early spring, and typically are fertilized once in February or March. Most shrubs make active growth in the spring and early summer, so we tend to fertilize them once around March or April. Lawns are allowed to green up before fertilizing in late March or early April and are fertilized again in midsummer.

Flowerbeds and vegetable gardens typically are fertilized just before planting. Fertilizing flowers and vegetables is especially important. We want maximum performance from these plants, and fertilizers help make that happen.

Not everything begins to grow in spring. Louisiana irises, for instance, are at the end of their growing season in spring, and are dormant over the summer. They begin to grow in October, and that is the time to fertilize them. Trees and shrubs, on the other hand, are going dormant in fall and should not be fertilized.

You do not need separate fertilizers for all of this, and there is no one best fertilizer. For landscape fertilization, choose a general purpose granular fertilizer that has an analysis with a higher first number, the lowest number in the middle, and a third number in between the two, such as 15-5-10. The exact numbers are not critical.

You also wonder how often to fertilize, and this depends on what you use. The label of the fertilizer will tell you the rate (amount) of fertilizer to apply and how often it should be used. Soluble fertilizers, for instance, are applied every two weeks during the growing season. Granular fertilizers are generally applied about every six to eight weeks. Slow-release fertilizers work for months, so one application is generally all that is needed for a growing season.

Dan Gill is a horticulturist with the LSU AgCente.r Have a gardening question? Email Dan Gill at dgill@agcenter.lsu.edu

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