Lawn fertilization in late autumn is the most important of the year

With the arrival of cool autumn weather, the need for certain lawn and gardening tasks slows down and eventually ends when the ground freezes. But the cool weather and shorter days in late autumn don’t signal the end of the need for some lawn maintenance. In fact, late fall is one of the best times of the year to upgrade a lawn stand, when an investment of time, sweat, and some lawn care products results in healthier, greener, denser lawns next to spring.

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Autumn is the time when the grasses of the cool season recover from summer stress such as drought, heat and disease. It is also the time when lawn grasses begin to store carbohydrate reserves in the stems and rhizomes. These carbohydrate reserves help grass plants fight injuries and illness in winter and, more importantly, serve as an energy source for roots and shoots to grow the following spring.

Mike Hogan

Fertilization in late autumn ensures better winter coloring of the stand, faster flowering in early spring and an increase in root growth. Late fall fertilizer applications are more important to lawn grass health than spring applications, which are more common practice by most gardeners.

When to fertilize the lawn in autumn

The best time to apply lawn fertilizer in late autumn is usually November or early December, when the grass is no longer actively growing or has slowed down enough to not need to be mowed. Although the leaves and shoots of the grass plant have stopped actively growing at this point, the roots are still actively growing, allowing the plant to make maximum use of the fertilizer available in the soil. It is customary to apply fertilizer after the last mowing of the season and fertilizer applications should never be made when the ground is frozen.

Now is the time for lawn care that will pay off next spring and summer.

Choosing a lawn fertilizer mix

While the best way to determine exactly how much fertilizer is needed is through a laboratory soil analysis, most cool season grasses require 2 to 4 pounds of nitrogen per 1,000 square feet per year, and a majority of that nitrogen should be broken down into two Case applications; one in September and the other in November.

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A complete fertilizer with a high ratio of nitrogen (N) and potassium (K) is essential for root growth, winter hardiness and disease resistance. An ideal autumn fertilizer mix has a nitrogen to phosphorus to potassium (N: P: K) ratio of 24-4-12. When buying lawn fertilizers, you may find that some manufacturers have completely removed phosphorus (P) from some fertilizers due to overuse and the environmental damage caused when excess phosphorus finds its way into surface water such as streams and lakes.

Late autumn fertilizing of house lawns will result in a healthier stand next spring.

Weed control in the autumn lawn

November is also an excellent time to control many annual weeds in the lawn. Annual winter weeds such as purple dead nettle, haircress, henbit, and chickweed are currently in the small seedling stage, and treatment with a herbicide at this stage of growth can be an effective way to clear them from the stand before they bloom next spring.

Other annual weeds like dandelion, plantain, and even clover can also be controlled in late fall, when these plants are still actively growing.

Lawn herbicide application is best done when daytime temperatures are 50 degrees Fahrenheit or more. So keep an eye on the weather forecast when planning applications in late fall. Always use the ester forms of lawn herbicides in the fall, as these formulations are more effective than amine forms in cooler weather.

So choose this fertilizer spreader in the back of your garden shed and supply your lawn with some nutrients at the time of year when it is most needed – in late autumn and winter.

Mike Hogan is an associate professor at Ohio State University and a senior lecturer at OSU Extension.

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