Fertilize Lawns in the Fall

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I read that it is very important to fertilize your lawn in the fall. Could you explain why fall lawn fertilization is so important?

I consider the fall application more important than the spring application. Fall lawn fertilization keeps your lawn dark green all winter. It does this without stimulating growth, so you don’t have to mow any more than usual.

The grass takes up the nitrogen in the fertilizer and stores it in the crown and roots as amino acids. This nitrogen is immediately available to be used for new growth in the spring. It gives a smooth even growth response which does not stimulate rapid spring growth that requires more frequent mowing in the spring.

It allows you to wait until May or early June to apply a spring fertilization. This late spring application lasts through the summer if it contains some slow release nitrogen.

I prefer a lawn fertilizer which has a 3-0-2 balance of nitrogen-phosphorus-potassium. A typical formulation would be 24-0-16. An organic lawn fertilizer would be more like 9-3-6. The numbers do not have to be exact, but approximately in this ratio. Special lawn fertilizer formulations for fall application (usually called “winterizer” blends) usually have the higher potassium amounts. I like to have at least 1/3 of the nitrogen in a slow release form since nitrogen is very soluble and some leaches below the grass roots. The most effective slow release mechanism is coating or encapsulating some of the nitrogen with sulfur or a poly coating or both. There are also chemical nitrogen compounds such as methyline urea which are slow release. Check the “guaranteed analysis” to see if some of the nitrogen is slow release. The analysis label will also list iron, which is also helpful, even in small amounts. I usually apply the same fertilizer in both fall and spring. Liquid applications by lawn care companies do not last as long as granular fertilizer. They are satisfactory as long as they are applied more often.

If you have broad leaf weeds in your lawn, a weed and feed product is a convenient way to apply both fertilizer and weed killer at the same time.

Apply lawn fertilizer any time from now until late November.