Fall Fertilization for Established Trees and Shrubs

New research indicates fertilizing trees and shrubs about a month after a killing frost is ideal

I love gardening for a lot of reasons. I enjoy the fresh air, physical activity, mental stimulation, constant change and more. The fall season is a great time to become reinspired to get outside and do some important maintenance in the garden and landscape. Big dividends in future seasons are the added reward.

One of my favorite parts about gardening is that I’m always learning. Fortunately, I learned something recently that changed my approach to when I fertilize established trees and shrubs.

For most of my gardening life, trees and shrubs that needed a nutrient boost got their annual fertilizer application in early spring, right before active growth began for the year. This timing has been the generally accepted practice by gardeners and experts everywhere for years. And although early spring is a good time, new research indicates there is an even better time.

Contrary to traditional wisdom, many experts now consider late fall, or about a month after the first killing frost, to be the ideal time for applying fertilizers. We now know plants utilize nutrients throughout the year in different ways.

In the past, the most common reason against fertilizing in the fall was the fear that plants and trees would put on new growth if unseasonably warm weather returned, only to be burned or damaged by imminently colder temperatures.

They key is to understand the difference between early fall and late fall timing. If you fertilize in late summer or early fall, when temperatures are still warm and plants are still actively growing, it is likely new growth could occur and damage to tender new foliage could be the likely result.

The rationale for late fall fertilization makes sense when you understand why. At this time, deciduous trees and shrubs have lost their foliage for the year and active growth of plants and trees has slowed. Rather than put on new foliage growth, the roots of established trees or shrubs take the nutrients from the soil and apply them to important health-promoting functions, such as disease resistance and root development. The excess nutrients are stored in the roots and become immediately available when needed for new growth in spring.

However, keep in mind, not all established plants and trees are candidates for a regular fertilization program. I always suggest a soil test be obtained through your local county extension office. Simply gather up a representative soil sample around the area where your trees and shrubs are growing. Be sure to inform the extension service you would like to have the soil tested for this.

The report will let you know what nutrients may be lacking in your soil for optimum growth. The report will also suggest the proper type and amount of nutrients to add.

A common mistake, and not just with trees and shrubs, is to assume fertilizer can and should always be added, and if a little is good, more is better. Nothing could be further from the truth. Excess nutrients are wasted and can end up contaminating the soil, and the environment beyond.

Plants and trees are far more sophisticated then we give them credit. In simple terms, they have built in clocks, timers, calendars and monitoring systems that don’t require our meddling nearly as much as we think, just like with fall fertilization.

About

Joe Lamp'l is the Host and Executive Producer of the award winning PBS television series Growing A Greener World. Off camera, Joe dedicates his time to promoting sustainability through his popular books, Compost Confidential blog, podcast series, and nationally syndicated newspaper columns. Follow Joe on Google+

Comments

  1. Robert says

    Do you have a recommendation for a fertilizer that I can buy from the local home improvement store? I just finished applying a winterizer to my yard which is around all the trees we have. Is that sufficient? I also have some shrubs that do not get this and I need to treat them separately.

    Thanks for any advice you may have.

    Robert

    • says

      I would use an all-purpose fertilizer around trees and shrubs. Such products have equal amounts of each of the primary nutrients (nitrogen, phosphorus, and potassium). Examples that you can find in box stores are: 10-10-10, 13-13-13, etc. The higher the number, the more of each nutrient is contained in the package by weight. Winterizer fertilizers designed for lawns are typically too weighted towards the first and last numbers in the analysis, so the balance is lacking. The bottom line is, that although trees and shrubs will also benefit from the winterizer, if the first number in the analysis is too high, you could stimulate too much late season growth, which you don’t want to do. For future applications, go with a balanced, all-purpose formulation for trees and shrubs.

    • says

      There’s no question they work for the area where the fertilizer is able to reach the roots. I like this method to keep the fertilizer contained to reduce runoff and deliver a steady supply of nutrients over time through controlled release. However, my issue is that the fertilizer is very contained around the area of the spikes. How far does the fertilizer spread? Not very. Tree and shrub roots go around the plant 360 degrees, yet the spikes can’t cover that much area, especially on trees. Most feeder roots are close to the surface. And how far out do the tips really go? You can guess but if you are too far out or too close, you miss the tips of the roots that take up the water and nutrients. It’s a guessing game. If it were me, I would broadcast the fertilizer around the root zone, out to the drip line and beyond. Yes, you will use more fertilizer, and there will be runoff (which I don’t like), but if you are trying to fertilize trees and shrubs, you need to get the nutrients to wherever the roots are.

  2. Dave B. says

    Joe,
    Is it safe to give the trees a healthy dose of compost tea around the drip line/circle of a tree? I’ve been doing this when I spray my lawn with compost tea and it seems my trees (all young trees) are responding well.

    Thanks for your help,
    Dave

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