I have become an expert on mammalian and avian intervention in the garden -- rather a dubious distinction. This year marks the fifth annual ritual that pits me against either four-legged or two-legged creatures, mostly of the domestic persuasion.

This year, it's the chickens. In past seasons, the invasion has been by the pigs, or the goats, the occasional peacock, one brief battle with rabbits, and, inevitably, the uninitiated visitor who can't tell the difference between a two-foot pokeweed and four-foot pea vine.

If you haven't been to a fencing store lately, you'll be surprised at the vast choice available. Some is lovely, but nonfunctional for keeping animals and small children out of the garden, and some is very functional but pretty hideous. Your choice will depend on what it is you're trying to keep out. In almost any environment, the biggest problems are likely to be dogs, who love to dig, humans, and the occasional wild creature, like a raccoon or a few rabbits. I don't consider cats to be a problem, unless you happen to harbor a particular dislike of them. As the host of perhaps a dozen or more cats, I find them rather comforting company in the garden, and they will go so far as to catch the occasional mole or mouse that might do some damage. The minor digging they may indulge in hardly hurts.

Choose something that is tall enough to discourage a dog from jumping over, and unapproachable enough to keep kids from climbing. A picket fence might be just what you need. But a rabbit could easily negotiate that, so, if this is likely to be a problem, you may have to line the inside bottom of the the fence with two-foot chicken wire. I find that a good fence around a garden will discourage most creatures, as much psychologically as physically.

Once the fence is up, it has many excellent uses, one of which is to plant it with roses, which would further discourage anybody from scaling it. Inside the garden, the fence makes a good support for vine crops, an attractive backdrop for hardy perennial flowers, and generally can function very nicely in landscaping. It also encloses the garden to create a small world in which escape is complete. And finally, a fence tends to keep encroaching weeds out.

Raccons, who are likely to be able to climb just about anything if they want to get into the garden badly enough, harbor a well-known aversion to punk rock and Musak, so leaving a radio on softly (so as to not bother your neighbors) at night generally discourages attacts on ripening corn, the racoon's favorite food.

QUESTIONS AND ANSWERS: Somehow, a few years ago, a few chrysanthemums sprang up in my vegetable garden. Within a very short time my garden was full of them. Last year I spent a week (truly!) carefully removing each and every (I thought) trace of the roots. I failed! All summer long I would find, and carefully remove, new shoots. It seems that only the tiniest piece of root is necessary for a new plant to spring up.

My problem is this. The root system is so thick that these "flowers" virtually choke the life out of my vegetables. Is there any chemical or any other treatment that will destroy those cursed chrysanthemums but leave my garden fertile for vegetables?

My initial reaction to your letter is that you could be a very rich person. Some horticulturist would pay you a small fortune for chrysanthemums that have the spreading habit you describe. Which makes me think that this is not a chrysanthemum at all but a weed, perhaps originally some type of wild bird seed, that bears a resemblance to mums. What it is, however, is irrelevant to the problem at hand.

While you could in all likelihood find some chemical that would wipe out the pesky weed, I'd be leery of using any in a place that is to produce food. I hate to sound like a broken record, but I have found that mulching heavily does eliminate even the worst weeds. In your case, an excellent mulch would be black plastic, which, even though unattractive, is very effective not only in keeping weeds out but in retaining moisture and heat in the soil.

Black plastic purchased at a plant store can be expensive for a good-size garden. But the same stuff can be purchased through a feed store as "hay tarp," for a lot less money. The smallest size that I have seen is 16 feet by about 25, and two years ago I paid about $15 for one.

Spread the plastic on your garden, anchoring it with dirt, rocks, or anything heavy that you can later remove once vegetable plants get large enough to anchor the plastic by themselves. Some people will cover the plastic with hay or grass clippings just to make it look better. When you get ready to put your plants in, just punch holes in the plastic and insert them. Any weeds coming through the holes alongside the plants will be easy enough to pull as they come up. For rows that you want to seed, such as spinach, peas, beans, and others, cut the plastic in strips, lay them side by side a few inches apart, and seed the soil. Between the rows, the black plastic will keep the weeds from sprouting, and again, in the rows themselves, any weeds that come up should be fairly easy to eliminate when still quite small. Over a season or two, the plastic will choke out your weed's root system, and, hopefully, eliminate it permanently. Make sure you use black pastic, and not clear. Clear plastic functions more as a good hothouse that will encourage a lot of weed growth.

BROCCOLI BONANZA: It's not too early to put in broccoli seedlings, which are much cheaper if you can buy them by the bunch. I picked up a bunch at the feed store last week. There were supposed to be a minimum of 35 plants in the bunch, and I actually got 50, so they can be a real bargain. More commonly available are 12-packs, which, as the name implies, consist of 12 individual seedlings each in its own growing medium. These will often be larger and more vigorous than the bunches, and are thus more popular, but they are quite a bit more expensive.

Regardless of the form in which you buy your seedlings, they must be thoroughly moist, if not soaking, before you plant them -- the roots, that is. Bunches should be placed in a small bucket or bowl so that they are about two-thirds submurged, tops above water, for an hour or two. Likewise, seedpacks are submerged, so that the soil is well covered, for just a few minutes, until the pack itself is very wet and heavy with water. This practice not only insures that the plants won't suddenly droop over from lack of water in a few days, but it makes removing them from their container much easier, if a little messy.

If you have tilled your garden, or if, like me, you don't till but prefer to simply mulch and your soil isn't packed down, there's almost no digging involved in planting broccoli seedlings. Take a sharp stick -- I use a stake -- and make holes in the ground about four inches deep, at least 18 inches apart. Take your seedlings and drop them into the holes so that they are almost entirely in the ground. Pack the soil firmly around the seedling, closing up any air pockets, and bringing soil or mulch up around the top of the seedling so that only the uppermost leaves are showing above ground. As with many annuals, roots will grow right out of the stem, giving the plant vigor and extra support. Later, as the seedling grows, you will side-dress the plant with more mulch, allowing additional roots to develop from the stem. Side- dressing will also help the plant conserve moisture.

Ideally, you want a gray, overcast day to transplant seedlings of any kind. But I find that this can be achieved with good success on a sunny day, too. Just be sure to check your seedlings daily for signs of drooping, atleast at the beginning. Once you see new growth beginning, you can relax. A word of caution: Don't try to crowd broccoli. I am a great advocate oft mu crowding plants if you have excellent soil, but broccoli won't stand for it. Plants that don't have room to spread their leaves will not produce good heads, and will stunt each other.

STARTED SEEDLINGS: If you've started your own seedlings, be careful about putting them outside during the days in this rapidly changing weather. It's a wonderful theory to let the little things enjoy a bit of afternoon sun on a warm day, but if you don't get home until quite late, or you forget them, which is what I did -- Zap! on a cold night. It doesn't take much. Temperatures didn't even get down to freezing, and still my melon and tomato seedlings perished. The broccoli survived without a scratch. Hardening off seedlings is important, but try to do that in an indoor room, where if you forget them, they'll still survive. Older seedlings, that have established a pretty good set of leaves and a vigorous stem, are less delicate, but still should be treated with special care until ready to go out permanently.