A persistent rooster has reminded me that this is the weekend to repair garden fences. He's been into the garden nearly every day, and I'm afraid he's eating the peas -- or at least soon will. I'll probably put him into the stew pot, but for now, he's serving the purpose of reminding me once again that a good fence around the garden is all too often the difference between reaping the rewards of your hard work and spending a summertime of frustration.

Even off the farm, fencing is important for keeping out gophers, rabbits, raccoons, deer, dogs and humans of all sizes. I'm thinking of the vegetable plot, mainly, although you may find that young flower seedings, too, can become breakfast for the local rabbit family. To keep out wildlife, you need a good, taut fence with gaps or openings no more than two inches apart. While deer can jump almost anything, they're less likely to jump over a fence than crawl under loose wire.

Cost, of course, must be taken into consideration when deciding what sort of fence to put up. The least expensive materials are metal posts and chicken wire. You'll have to put in some wooden posts, especially at the corners, if you use chicken wire because when the wire is stretched, metal posts will pull out of the ground if they're the sole means of support. While relatively inexpensive, this kind of fencing is also the shortest-lived.

Electric fence, while cheaper than chicken wire, is not recommended for keeping out small animals, which can easily crawl under it. Besides, young children can get zapped on it, too.

An alternative to short-lived chicken wire is vinyl-coated turkey wire, which is more expensive but will last longer and, in the long term, be cheaper. A third possibility is prefabricated panels, either solid wood or closely set slats, such as picket fencing, available from any fencing center.

This last choice is the most attractive, most durable and most expensive. Using conservative estimates, figure on chicken wire lasting four years, vinyl-coated turkey wire six, and wooden panels 10.

Regardless of what you choose, bury the bottom of the fence 12 inches into the ground to prevent rabbits and groundhogs from digging underneath. The fence should be five feet tall altogether, so that it rises four feet above the ground level.

For such a fence, you'll need posts every eight feet. Half-round cedar or black-locust posts are ideal since they'll last 20 years before having to be replaced; an alternative is a high-quality pressure-treated pine. You can expect these to last all of 10 years and perhaps longer. Metal posts will need to be replaced after five years or so. If you choose wire fence, incidentally, go for the heaviest gauge you can find. The extra cost will pay off in the long run.

Metal posts can be hammered into the ground from the top of a stepladder using a sledgehammer. Wooden posts require holes to be dug to a depth of one-third the length of the post. Thus, a four-foot fence will need six-foot posts set two feet into the ground -- about an arm's length.

The best tools to use for digging post holes are a digging bar -- a 25-pound metal bar with a flat blade at one end for loosening soil and a round flat surface at the other end for tamping the soil down around the post -- and a clamshell-type post-hole digger, which allows you to get right down into the hole without having to make it excessively wide. The digging bar and post-hole digger can be rented. If you buy them, they'll cost about $30 for the bar and $17 for the digger.

If you're working alone, it will take you about 20 minutes per hole for digging and another 10 to set the post for wooden posts -- working hard and in fairly good soil, free of stones and rock.Hammering in metal posts takes no more than five minutes per post. Because it's so important to set posts so they're absolutely vertical (this requires the use of a level), the job is much easier with two people.

The bottom of the post hole should be as flat as possible. Set the post and throw in as many rocks as possible, then start adding soil, tamping it down every few inches with the digging bar. Tamp it hard and all around before adding more soil. This way it'll be rock hard when the job is done. The final layer of soil at the top should be slightly higher on one side so that rainfall will run off the post, rather than down into the footing.

There are a number of gadgets available for stretching a wire fence -- a problem that you'll avoid if you go for wooden panels. After setting posts correctly, stretching the wire is the most difficult and the most important part of building a fence. If the fence sags, it will be easy for an animal to crawl under it. After just a few months, it will be loose enough to come out of the ground. Take it from someone who's had personal experience with sagging fences -- they also look awful. Eventually, they bend down so much from winds and rain that a rabbit, which can jump two or three feet, will have no trouble getting over.

I've found a simple pulley to be adequate for stretching short lengths of wire. Clamp a couple of 2 X 4s like a sandwich to the end of the wire and fasten them together with carriage bolts. Set the entire device perpendicular to the ground, like the posts, and then pull on the pulley. Instead of using a pulley, you can also stretch the fence with a garden tractor or car, using heavy ropes or chains. Stretch the wire in only one direction at a time. When you have one side stretched, fasten it very firmly to a wooden corner ost with heavy fence staples. Metal posts come with a bag of clamps or teeth to hold the wire on.

This is by no means a complete guide on how to build a fence. Your fencing supplier will have illustrated instructions on how to do the job. But at least you know now what you're up against. PLANT THIS WEEKEND -- Lettuce, spinach, radish, carrots, turnips, beets, peas, shrubs, fruit trees, ornamental trees, boxwood. PLOTTING GARDENS IN FAIRFAX -- Registration for 400 garden plots will be held Saturday morning from 7 to 11 at Green Spring Farm Horticultural Center, 4601 Green Spring Rd., Annandale. Residents of Fairfax County or Fairfax City may farm these plots, which are spread around 10 locations in the county. The plots measure 20' X 30', cost $25 for the season, are rototilled and ready to use. Call 941-5000, ext. 226.