The nonstop rain has grown rather boring, but one can't help but be grateful, up to a point.

Many seedlings that gained a foothold when the days were dry now are taking off and establishing very beautiful and very sturdy foliage, an important factor that will pay off in larger crops. However, many seedlings were washed out and a lot of plants, especially vines have been battered, causing blossom loss and fruit rot. Peas should be given ample support and propped back when they fall over. Rot should be trimmed away as soon as possible to prevent the spread of diseases.

Lack of sunshine may well have retarded blossom development. Just as important, bee activity is down on cloudy or rainy days, which often means that a plant will not be pollinated until the weather improves. So anticipate a later harvest even from plants that were put in quite early.

There's not much you can do about rotting in root crops if your drainage is poor. Just pull up rotten carrots, beets, turnips or onions, and plan on adding sand to the soil for better drainage.

Early blight on tomatoes may yellow the leaves. Just wait it out. The rain brings it on but damage should be minimal. Once the world dries out the problem will go away. Slugs enjoy this kind of weather, and the old method of putting out saucers of beer to attract and drown them may not work if there's going to be a downpour that night. Another method of getting rid of slugs is Jim Crockett's (Crockett's Victory Garden) tip of laying out a half-grapefruit rind, a cabbage leaf or square of plastic. The slugs, which come out at night, congregate under whatever you put down. Next morning, pour salt on the throng.

According to horticulturists at Brookside Gardens (advice is available from them Tuesdays through Thursdays at 949-8230), the most important aspect of dealing with rain in your garden is sanitation. Keep broken or spoiled vegetation clipped and disposed of.

Black spot on rosebushes thrives in cool wet weather. Spray roses with an all-purpose spray. Brookside Gardens also cautions that heavy rain will encourage dense growth in many perennials, reducing air circulation over the plant, which can bring on diseases such as botrytis blight. Pruning may be called for.

JUMPIN' JUNE: How often I hear, about this time of year, the remark: "Well, I'd like to have a garden, but it's really too late to plant anything."

The fact is that late June is by no means too late to start. There are many gardeners who begin planting seeds and seedlings in early March and continue right up into late September. These last planting would normally be designed for early spring crops, but there are things that can go in now for crops this year.

If you scrounge around hardware and garden stores, you are likely to find some that still have a six-pack or two of spindly tomato sets or peppers. Cabbages and brocolli plants may be another matter, but there should be a few summer squashes and cucumbers still to be had. At this time of the year, stores are trying to get rid of the last of their annual garden plants, so bargains are widely available. No one thinks twice about planting annual flowers now, so why not a few summer veggies?

In seeds, there is still time to plant corn, especially if you soak the seeds overnight before putting them in the ground. Beans are something that can go in right through next month. Pumpkins, winter squash, spaghetti squash and cucumber, all from seed, will do fine if put in now.

If the vegetable lot is not yet prepared, and this may take a weekend or two to accomplish, spend a couple of days on that, and plan on putting in a fall garden. July and early August plantings of peas, broccoli and cabbage (these last two vegetables are often available from nurseries later in the year), lettuce, turnips, beets and radishes will yield excellent crops in September and October. Rent a tiller and turn up a bed, perhaps four feet wide and eight feet long. Add some store-bought peat and manure, a little lime (don't bother with soil tests at this late date), and try a few packs of seeds. You are apt to have fewer weed problems late in the year, but you may have to do quite a bit of watering.

So, say you don't have enough space, or would rather grow flowers, or don't have the time to spend on the garden. But don't say that June is too late to plant.

FOREIGN FOODS: What do you do when a well-meaning friend brings home packets of vegetable seeds picked up abroad, and hands them to you for cultivation? In my case, these seed packs included radish- shaped carrots, French cornichons, pickling cucumbers and purple chard (with, of course, no English instructions).

The most obvious rule of thumb on foreign seeds is to consider the country of origin. If the climate is similar to ours, proceed as though they were local varieties: The carrots would go in along with other carrots, the chard about the same time as Swiss chard, and the cucumbers can be started indoors in early spring for outdoor transplanting after the frost date, or planted directly outdoors in late May or early June. Seeds from a tropical country are likely to be slow producers that thrive in hot weather. Seeds from cooler countries (Oriental vegetables are always a treat) are likely to be happy if treated like our brassicas.