When should you start fertilizing and when should you quit? The general rule of thumb is that plants need more available nutrients during the growing season and may not require much, if any, during their natural rest period, if it exists. Here in zone 8B in west central Louisiana our last frost date is around March 1st and our first frost date is about mid-November. We begin fertilizing hardy woody species and hardy winter and spring growing perennials during late winter or early spring (mid-February to early March) or up to about 6 weeks before their natural growth cycle begins. We generally want to stop fertilizing in time to give these plants a minimum of 6 weeks (slow-release fertilizers should be depleted by this point) for growth to begin to slow and for them to begin to naturally harden off before the first expected hard frosts or freezes occur which works out to about midsummer and should be done no later than mid-August. 

     Why does it matter? Vigorous tender late season growth can be susceptible to freezing temperatures even for what might seem the hardiest of species. The freeze damage can actually do much more harm than good splitting bark and causing die-back on what may normally be reliably hardy species. The wounds caused by split bark, stems, and foliage can also provide an avenue for pests and diseases to enter normally problem free, pest resistant plants. Often, the results are not aesthetically pleasing and you may have to “admire” your handiwork until spring new growth hides it if you don’t end up losing the plant(s).

     Plants that are happily growing may not need to be fertilized every year. Under optimum conditions natural soils in combination with appropriate organic mulches, mycorrhizal fungi, and soil microbe associations (about 95% of plant species are considered to have them) additional nutrients may not even be necessary.

    Containerized plants are solely dependent on the gardener for nutrients. Some potting soils may contain some nutrients initially but will eventually be depleted. These plants depend on us to provide the necessary nutrients in order to thrive.


Plant Type

Examples

Begin Fertilization as Early As

End Fertilization

Reliably Hardy Woody Trees, Shrubs, and Vines

Azaleas, Camellias, Hollies, Vitex, Honeysuckles, Akebia

Late winter or early spring

Midsummer or 4-6 weeks prior to the end of the growing season. About mid-August for zone 8

Spring or Summer Growing Annuals, Perennials, and Biennials

Impatiens, Coleus, Petunias, Baptisias, Salvias, Hardy Hibiscus

Late winter or early spring or at planting time for annuals.

Midsummer or 4-6 weeks prior to the end of the growing season. About mid-August for zone 8 

Tropicals grown as Die-back Perennials or “Tender Perennials”

Angel Trumpets, Lantanas, Passionvines, and other marginally hardy species

Once spring growth commences. March to June depending on your zone.

Midsummer or 4-6 weeks prior to the end of the growing season. About mid-August for zone 8

Fall and Winter Growing Perennials, Annuals, and Biennials

Daffodils, Louisiana Irises, Phlox

Fall or early winter

Mid to late spring or once flowering has completed

Tropical Plants grown under warm tropical conditions

Tropical Hibiscus, Passion Vines, Crossandras

During their growing season which may be year round for many tropicals.

About 4-6 weeks before the end of their natural growing season if there is one