Don't think you can send off for a plant today and get it 10 days from now. In general, allow six weeks or two months.

You may read in a great rose nursery's catalogue that orders received after Oct. 15 will be held for spring shipment. You might think, therefore, that orders received before that date would be shipped the same fall. If you try it, you are likely to discover that roses to be shipped in late November were ordered by far-sighted gardeners in July. You will also probably discover that a rose ordered Oct. 15 will not be sent in the spring, either, because the variety you want has been sold out for months.

Nurseries raise varieties of plants to fill what they hope will be a reasonable demand. If you run a nursery and notice that year after year you sell about 100 plants of a particular rose, you would be foolish to raise 1,000. But if a particular variety is much touted by the gardening press, the demand may be exceptionally heavy that year, and the plants simply will not be available.

I suppose the nurseryman then raises 200 for the next year, by which time gardeners have forgot all about it and turned to something else and he's stuck with surplus stock.

It's obvious, if you think about it (as so many things are, but who thinks?) that two-year-old field-grown roses cannot be suddenly pulled out of a hat, no matter what the demand is, and neither can 40-year-old specimen pear trees or, for that matter, seeds of a particular strain of pansy.

One year I mentioned a most beautiful minor spring bulb, Brodiaea uniflora, with perfumed star-shaped sky-blue flowers rising on eight-inch stems from dense tufts of garlic-scented leaves. I said no more than I believed, that life is not worth living without it, and an august editor ordered a batch. He received a form letter saying the bulb could not be supplied, as some fool garden writer (they didn't say fool writer, but that's what they meant) had revved up an abrupt and unprecedented demand for the thing. You might wish to note right now that this lovely April-blooming small bulb should be ordered next July for planting next October to bloom in April 1989.

Something gardeners seem not to be aware of is that if a plant is sold out this year, it is only 365 days until next year, but I know gardeners (I am one) who have fumed and fidgeted for a decade for a particular plant, faithfully ordering it far too late every year. The pleasure of snarling at a nursery does not make up for the sorrow of not getting the plant. But I am sure we all get the point now.

The first and largest of the flower shows of the East Coast (and of America, surely) is the Philadelphia Flower Show, March 6 through 13. Tickets are $8.50 at the door of the Philadelphia Civic Center (34th Street and Civic Center Boulevard) or $7.75 in advance. Information on group sales, travel and lodging can be had by phoning (215) 625-8250, or by writing the Pennsylvania Horticultural Society, 325 Walnut St., Philadelphia 19106. If those who plan to attend will tear out this little note now it will save trouble later, as I well know from past years.

The show this year will have massive displays of orchid plants in bloom from some of the world's premiere orchid collections, a display that show managers call unprecedented. Apart from the main display of orchids there will be a number of gardens created on the spot beneath the acres-wide roof, many of them costing more than $10,000 to install. There will be the usual endearing displays of pot plants -- Uncle Will may well display his old barrel cactus, etc., and great space will be devoted as usual to flower arrangements and tableaux.

Having hit you properly over the head about ordering plants in good time I shall now leave you at peace to admire the progress of your snowdrops. If your snowdrops are not blooming, possibly it is because you didn't plant any? Again, order them from bulb dealers next summer for delivery in September, if possible. Even better, buy a clump in full leaf, just after the flowers have finished for the year -- say March 1 -- if you should see any at farmer's markets. I have never known where to buy snowdrops in clumps, in full growth in March, but that is the best way to acquire them. Otherwise you have to go with the dried-off bulbs of early fall, and many of these will not grow. Plant 100, and hope 50 will grow.

Some snowdrops, especially Galanthus elwesii, are collected in the wild from dwindling and threatened stocks. I do not think we want to decorate our gardens at the cost of extinguishing the wildflowers of Turkey and other nations. Tell your dealer you do not want to buy bulbs illegally imported from wild stocks, and ask for his assurance that your bulbs next fall -- wild tulips, snowdrops, small daffodils, etc. -- were all raised in nurseries.