Mother's Day may be as American as apple pie and the Fourth of July.

But the flowers you buy for our mother next week may not be.

Just as foreign firms have invaded the automobile, steel an shoe industries, foreign flower growers have discovered that the United States is very attractive market for selling their goods.

And while the imports aren't threatening a linchpin of the U.S. economy--Americans spend about $455 million on fresh-cut flowers each year--the industry says it is a $5 billion business when such items as plants and accessories are included.

"If you're buying pompons or carnations, the chances are better than 50-50 that you'll get a South American pompon or carnation," says Howard "Suds" Gilliams, manager of Claymore C. Sieck Inc., one of the Washington area's largest wholesale florists. "To some degree, the same is true of a rose."

Colombia accounts for the largest share of imports--so much so that flowers have become that nation's second biggest export, behind coffee. But fresh-cut flowers also come in large numbers from Israel and the Netherlands. A growing numbers also are beginning to trickle in from Mexico and South Africa.

The imported flowers are available everywhere--not only in the local small florist shop, but also at the street corner stands and in the supermarkets that have entered the floral industry in a large way since the mid-1970s.

According to U.S. flower growers, numbers tell the story. In 1972, imports accounted for 10.5 percent of all the pompon chrysanthemums and 8.3 percent of all carnations sold in the United States. Those two flowers are among the largest selling items, says James Krone, executive vice president of Roses Inc., a national trade association whose members produce 80 percent of all greenhouse roses grown in the United States and Canada.

Last year, Krone notes, 55 percent of all pompons and 59 percent of all carnations bought in the United States were grown elsewhere. "In a relatively short period of time, the ownership of pompons and carnations has moved offshore," Krone says, adding that the same trend is beginning to appear in the case of roses. in 1982, nearly 15 percent of all roses purchased here were imported.

The importers "are putting a dent into us an scaring us half to death," says Krone, who says that in the six months since last September, his organization lost 12 members as a result of the rising imports.

Krone's organization sought special protection from the International Trade Commission in 1979, but the commission rejected the petition, finding that imports did not constitute a serious threat to the industry.

As a result, Roses Inc. in now trying to win sympathetic action from Congress, where it is seeking legislation to double, at least, the tariff for fresh-cut flowers.

This move, however, is being opposed by many retail florists who argue that the only hope they have of keeping prices down and business high is through imports.

Overall, the sales of imported flowers last year totaled about $140 million--almost twice the $76 million sold only three years earlier, according to estimates by the International Trade Commission. Imports accounted for nearly a third of all U.S. cut-flower production.

In the Washington area, Gilliams estimates that about 20 percent of all flowers are imported.

Ted Johnson, manager of the Gude and Sons Co. wholesale division, says no more than 10 percent of his business is in imported flowers. Most of the store's goods come from California, he says, largely because they are easier to get. "When I order from California, I can order midday and have the them the next morning; when I order from Holland, I have to order early Tuesday in order to get them by midday Thursday."

But Dennis Paul, president of Paul's Wholesale Florist, says about 40 percent of his sales of perishable items are imported. The company was reluctant to go abroad, but had no choice, Paul said, because "we found there were less products available in the U.S. We still have a lot of domestic flowers grown in this country and we are still working with these folks and want to continue," he says. "But expansion in this country has stopped and other countries are filling in the gap," especially at holidays like Mother's Day, when demand is high.

According to a recent report by the International Trade Commission, the number of firms producing fresh cut flowers dropped from 5,000 in 1976 to 4,500 in 1980, with the sharpest declines in the number of producers of carnations and chrysanthemums. In fact, the decrease in carnation growers has been so sharp that the Carnation Society of America has been disbanded.

The ITC concluded that the "decline in the number of producers of standard and miniature carnations and pompon chrysanthemums is believed to be in part due to increased import competition in the domestic market for their products."

One reason: given the sharp increase in energy costs, the South American countries, which are near the Equator and don't have to worry about fuel bills, can grow flowers much cheaper and easier.

Nor can they compete with countries where labor rates are far lower than than in the United States, notes Don E. Hook, a Pittsburgh grower and wholesaler who is on the board of directors of the Society of American Florists's wholesalers council. Floral experts say the hourly wage of about $6 in the United States is equal to the daily wage in Columbia.

But the decline in U.S. growers is attributable to more than competition.

Finding that their greenhouses and farms have now become valuable real estate property as cities have expanded to what was once farmland, many producers are now selling out for big sums to developers, especially in the Northeast, Florida and the Midwest.