Gardener's Math

The area of a lawn or garden in square feet is one of the most useful--and used--measurements of all. For example, fertilizer application rates are often given per 100 or 1,000 square feet. The area of a lawn or garden is easy to determine--especially when its shape is fairly regular.

The area of a rectangle is calculated by multiplying the length times the width. In this bed, which measures eight feet by four feet (figure B), you would have an area of 32 square feet.

A triangle is basically half a square or half a rectangle, so use the formula for determining the area of either a square or rectangle and divide by two.

Most lawns and gardens aren't shaped like perfect squares, rectangles, triangles or circles, but you can still approximate the area of an irregularly shaped garden using the same formulas by fudging a little.

You can fudge the other formulas in much the same way; although the answers you get will be approximate, they'll be close enough. These calculations are important; only when you know the areas of your lawn and gardens will you be able to apply fertilizers and other garden products with precision.

For example, if the application rate is 5 pounds per 100 square feet, and if you have 200 square feet of garden, you'll need 10 pounds; if you have a 50-square-foot garden, you'll need 2-1/2 pounds.

But Paul James recommends using less. "Since I firmly believe that we all use twice as much fertilizer as plants really need, I always cut the amounts recommended by the manufacturer by half. Your plants will be happier as a result, and you'll save some money."

Whatever you do, says James, don't exceed the recommended application rate. "Don't assume that if five pounds is good that 10 pounds would be better, because that's a formula for disaster as far as your plants are concerned--and a big waste of money."

Perimeters:

Volumes:

Gardeners also need to know volumes--especially when it comes to potting plants in containers, because potting mixtures are ordinarily sold in quarts or cubic feet. If you feel very mathematical, you can compute the volume of a cylinder: multiply pi times the radius times the height. But there's an easier way--use these guidelines as benchmarks: a one-cubic-foot bag of potting mix is roughly 30 quarts, and it will fill 18 six-inch pots, three 10-inch pots and one 14-inch pot.

Mulch is another product sold by volume--usually in bags containing three cubic feet; however, gardens are measured in square feet. To know how many cubic feet you need to cover a certain number of square feet, you need to understand what a cubic yard is. One cubic yard of mulch will cover a 100-square-foot garden to a depth of three inches. One cubic yard equals 27 cubic feet. You would need nine three -cubic-foot bags to cover a 100-square-foot garden.

But just for simplicity's sake, think of easy-to-remember round numbers: for a 100-square-foot garden, you'll need 10 bags of mulch, or one bag for every 10 square feet. That will give you an extra bag of mulch--not a bad thing.

Dilution rates:

At one time, the dilution rate for various gardening products sold in liquid form were hard to understand and very impractical for the home gardener. For example, if the instructions called for eight ounces of product per 25 gallons of water, you had to figure out how much to mix in one gallon of water, or even one quart of water. Now things have changed. Nearly all products include dilution rates for smaller quantities--typically one gallon--and many are pre-mixed so there's no need for diluting.