Garden Math Made Easy (yeah, right!)

Outdoor chores frequently involve mathematical calculations. How much mulch do I need for the garden bed? How do I know when I have applied an inch of water to my lawn? And most importantly, how much beer do I need for tonight's barbecue party? 

For those of us who are number-impaired, here are some quick tips:

Keep a few measuring devices handy in the garage. 

They include: a retractable tape ruler, preferably one that extends 25 feet or more; a small, battery operated calculator; a bathroom scale; a set of measuring spoons; a measuring cup; a small container that measures tablespoons and/or ounces, such as the ones that come with cough syrup; and, a two-gallon plastic bucket.

Since some of these will be used to measure garden chemicals, be sure to label each for garden use only.

For those of you with big bags of lawn fertilizer (but small lawn areas), use the bucket and scale to weigh out the amount of lawn fertilizer to correctly apply. To calculate how much you need: Divide the weight of the full fertilizer bag by the bag's listed coverage area (in square feet); multiply that answer by the square footage of your lawn.

For example: an 18 pound bag of fertilizer with a coverage area of 2,000 square feet; and, a 750 square foot lawn (this is why you need the calculator). 18/2000=0.009.  750 x.009= 6.75 lbs. Weigh out between six and seven pounds of fertilizer for your lawn, accounting for the weight of the bucket on the scale.

 
Other handy garden formulas: 

• To determine the area of your yard, multiply the length by the width (both in feet). The answer will be in square feet.

• To determine the diameter of a circle (such as a tree trunk): circumference divided by 3.14. To measure the circumference of a tree trunk, wrap a fabric tape measure (or a piece of string) once around the trunk, about waist high.

• To determine the area of a circle: 3.14 times the radius squared. When measuring the area beneath a tree, the radius can be calculated by extending the ruler from the trunk to the drip line (the furthest extension of the tree branches).

• Approximately one cubic yard of mulch will cover 100 square feet with three inches of mulch. A more exact formula: Area (in square feet) times depth of mulch or compost you want to apply (in inches) divided by 324 will give you the number of cubic yards to purchase.

• 27 cubic feet equal one cubic yard.

• Three teaspoons equal one tablespoon. Two tablespoons equal one ounce. 16 tablespoons (eight ounces) equal one cup.

Lawns in the Central Valley and low foothills need about an inch and a half to two inches of water per week in the summer, divided into two or three waterings per week. 

To determine how much water your sprinklers are putting out: 

• Position 6 to 10 flat-bottomed, same sized containers around your lawn. Use drinking glasses, Tupperware, tuna fish or cat food cans; containers with taller sides will keep the water from splashing out. Put some in the greenest areas; put some in the areas that are struggling. 

• Turn on your sprinklers for 30 minutes. Then, measure the amount of water in each container. There should not be more than a quarter-inch difference among all the containers. If there is, readjust or add to your sprinklers to hit those areas that aren't getting as much water. If, on average, you are getting a half-inch of water per container during that 30 minute test, then you need to water your lawn for two hours a week in the summer, to put two inches of water on your lawn. In this example, you would water your lawn twice a week, for an hour each time. 

• You may need to adjust this timing if you see water streaming off the lawn. In that case, reduce the amount of time the sprinklers are on at any one time. Then, add a second cycle a few hours later. 

• It is best to water with rising temperatures, which in the summer, is from about 4 a.m. to 10 a.m. Earlier is better.

And if you see a newspaper article that mentions acre-feet of water and household use, do the math to check them.

* One acre-foot of water represents the needs of about four average California families, in the home and landscape for one year (average daily usage in California is 192 gallons per family; in meter-resistant, hot-summer cities such as Sacramento and Fresno, the total jumps to 280 gallons per family). 

An acre-foot covers 1 acre of land 1 foot deep.
1 acre-foot of water = 43,560 cubic feet = 325,900 gallons
1 cubic foot of water = 7.48 Gallons = 62.4 pounds of Water
1 million gallons per day = 1,120 acre-feet per year