This is a question I get asked quite often. Fertilizing in spring seems logical. After all, plants need energy to flower, leaf out and sustain themselves all through the summer. In the fall, however, they are getting ready for winter dormancy. So, why do they need energy if they’re “going to sleep”?
Confusion may have been created by some garden communicators and manufacturers who refer to fertilizer as plant food. It’s not. Fertilizer aids in the food making process much like vitamin supplements aid in our metabolic process.
Plants are hard at work over the growing season making food through photosynthesis. The food they make in the fall will be stored in their roots and distributed to other parts of the plant that need nourishment all through the winter.
Nitrogen, phosphorous and potassium, as well as the micronutrients plants need to make food, are all found in good topsoil naturally. The problem is that many residential soils aren’t that good. The topsoil is often removed during excavation and not returned when the house is finished. I even saw one case in which the contractor raised the grade in a back yard by spreading three layers of subsoil on top of the original topsoil. Subsoil is almost always deficient in one or more nutrients and the only way to replenish them is with fertilizer.
If you fertilized in the spring, the plants used those nutrients all spring and summer to make the food they needed during the growing season. Now, as they are in the midst of a full court press to make enough food for the winter, they need more nutrients. In addition to winter sustenance, the plants need to have enough energy in reserve to break their flower and leaf buds in the spring. Fertilizer is the source of those nutrients.
Granulated fertilizer is fine for lawns and perennial beds but trees and shrubs need accessibility to the nutrients quickly so we apply balanced fertilizer in a liquid form. A giant needle is placed in the soil right at the root zone and liquid fertilizer is pumped directly into the area where the roots can immediately begin absorbing the nutrients. It should be noted that all fertilizers must be in liquid form for plant roots to absorb them. That’s why you should apply granular fertilizer within 24 hours of impending rain or be prepared to water it into the soil by irrigation.
Sometimes, roots need some help finding water and nutrients in the soil. In that case, we’ll mix mycorrhizae with the liquid fertilizer. These beneficial microbes (bacteria and fungi) attach themselves to the roots and extend their reach. The mycorrhizae pass the nutrients on to the plant and the plant shares its food with the mycorrhizae. It’s called a symbiotic relationship
The answer to the title question is that plants need extra nutrients in the fall to “bulk up” for winter dormancy, much as bears and other hibernating animals need to binge eat before they lie down for their long winter nap.
In our mild climate, we should still be careful with the later application of fertilizer for some of the tropical or semi-tropical plants because it can stimulate new growth that will not be happy when the weather actually changes. Such plants do not seem to know much about getting through winter, so do not mind putting out new foliage when they should not.
It just occurred to me though that we do give palms, including the tropical sorts, micronutrients to help them through the cooler temperatures when such nutrients are not so available. It does not stimulate new growth, but helps those that are likely to yellow stay green.