Now’s the best time to apply slow-release fertilizer — and to figure out just how much to use.

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IF YOU FERTILIZED your lawn only one time per year, the best time to do it would be between the end of the second week in November and the end of the first week in December. That’s because in late November, grass goes through a metabolic change. Instead of producing top growth, food manufactured in the leaves is stored as carbohydrates in the roots. Feeding during this time period in late fall greatly increases the amount of carbohydrates produced and therefore results in a much deeper root system.

If you prefer to use organic fertilizer next year, feed your lawn in mid-October with organic food. But applying organic fertilizers this time of year isn’t generally effective, because it’s too late. Microorganisms in the soil must break down organic fertilizers to make the nutrients available to the plants. In cooler weather, the microorganisms are less active, and it can take up to 6 weeks for soil microbes to make enough nutrients available to increase carbohydrate production when it is needed in late November and early December.

At this time, it’s much more effective to apply a synthetic, slow-release fall and winter lawn fertilizer. The ingredients in synthetic fertilizer are instantly accessible to the plant, so adequate nutrients will be available to promote carbohydrate production at the time it’s most needed.

Make sure the fertilizer is slow-release, water it in and apply only the amount recommended on the label. Most fall and winter fertilizers contain iron, which is good for grass, but in excess it can jeopardize the health of kids and pets. Too much iron can also burn the grass and turn it black. That won’t only look bad, but will drastically reduce carbohydrate production, and actually cause the root system to shrink rather than grow deeper.

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The problem most of us have is figuring out how much fertilizer to apply. The label on the fertilizer bag will tell you how many pounds of product are in the bag and how many square feet the ingredients in the bag will cover, so in order to know how much to apply, you need to know the size of your lawn. If you don’t know it offhand, you’ll need to measure it. Rental shops usually carry measuring wheels, which make the task much easier, especially if it’s a large area.

If you have a square or rectangular lawn, simply multiply the length times the width to figure out the square footage. It can be more challenging, however, to measure an irregularly shaped lawn. If you have a teenager in high-school geometry class, challenge him or her with a “real-life” calculation; if not, the internet has many sites with instructions on how to measure and calculate the square footage of a lawn of practically any shape: a half-moon, triangle, circle or anything in between.

If you know the square footage of the lawn, and how many square feet the bag covers, it’s easy to figure out how much fertilizer to use. For instance, if the bag covers 5,000 square feet, and your lawn is 2,000 square feet, then you will need to use approximately 40 percent of the product in the bag.

Of course, you still have to figure out what setting to use on the spreader. The easiest way to get around that problem is to set the opening to a low setting to restrict the flow. Fill the hopper with the amount of fertilizer needed for the size of the lawn. Then apply the fertilizer by going over the lawn several times in different directions until all of the fertilizer in the hopper has been applied. This method gives even, accurate coverage.

Don’t forget to sweep up any fertilizer that landed on adjacent sidewalks, to make sure it doesn’t find its way into nearby storm drains. Finally, the most important thing to remember is to write the square footage of each lawn on the wall of the garage in indelible ink. That way, you won’t have to go through the whole measurement operation all over again.