Landscape and Nursery - Fall Fertilization Pros and Cons

The following is a good article on the pros and cons of fall fertilization from Andrew Ristvey from the University of Maryland.

Fall fertilization of landscape plants is truly a beneficial practice as long as certain guidelines are followed. This has been a contentious issue with experience on both sides challenging fall fertility. Physiologically speaking, even though the tops of plants have gone dormant or have slowed down, fall is an active period for roots. They are still growing and absorbing nutrients for next year’s spring flush until soil temperatures inhibit biological activity. All of next year’s spring buds will grow from stored nutrients attained this year. So the most effective application of fertilizers for next year’s growth is during late summer and fall of this year. However, because of dry and hot weather often experienced in Maryland towards the end of summer, optimal fertility times are towards the fall period.

Certainly, the most important growth factor for plants is water, and that was made evident this past mid-summer, when plant water stress was high. In the landscape, especially where plants do not have the luxury of irrigation, hot summer weather will inhibit plant growth. Fertilization should be minimized or stopped until better climactic conditions promote growth.

It is true that an over-application of nitrogen in fall can potentially awaken near dormant buds and expose plants to damage from frost which is just around the corner in our region. One relatively new study in the Journal of Arboriculture reviewed past research and looked at the effect of fall nitrogen fertility on cold hardiness of 5 landscape trees including Leyland cypress, crape myrtle and red maple in North Carolina. In most cases the researchers found no significant differences in hardiness of spring bud tissue with different nitrogen treatments for all the species. So there exists some evidence that fall fertilization does not reduce winter hardiness. But I have seen a heavy application of nitrogen in container grown plants increase vegetative growth in fall resulting in frost damage. However, those plants had not become completely dormant.

Most fall-based fertilizers are low in nitrogen and have higher ratios of phosphorus and potassium. Obviously, a serious fertilization program should rely upon a soil fertility test so that adequate amounts of nutrients are applied without risking toxicities or antagonisms (one nutrient overapplied can affect the availability of other nutrients). However, in general, a half rate of a low nitrogen or a 50% WIN combination in October should be considered as long as there is no longer any shoot activity. In container culture, a very low soluble nitrogen application (with excellent irrigation management to prevent nutrient runoff) may be acceptable periodically until temperatures fall below 55 °F, especially if your controlled release fertilizer prills no longer contain nutrients. Remember that fruit tree nutrition is based on leaf samples which should have been taken before the harvest. Fertilize according to those samples.

Information and photo from the October 9, 2009 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension http://www.ipmnet.umd.edu/09Oct09L.pdf