This Friday the 13th is really lucky for those interested in plants: The National Herb Garden, a major new attraction at the Aboretum, opens to the public.

It will encourage and help many to plant herbs in their own gardens, resulting in much tastier food for those who use them.

Until about the 16th century; herbs were strictly medicinal plants; then books about herbs began to include food and horticultural plants. Today, most definitions of herbs include a broad range of trees, shrubs and herbaceous plants valued for medicinal, savory or aromatic qualities.

The Herb Society of America defines herbs as plants that are useful to man, or as plants for use and for delight.

The National Herb Garden, a cooperative undertaking of the Arboretum and the Herb Society, occupies about two acres in the 444-acre Arboretum. Since 1965 a National Herb Garden has been a continuing project of the Herb Society.

"A little more than a year ago, the Herb Society presented a $200,000 check to Secretary of Agriculture Bob Bergland for the garden," says John Creech, director of the Aboretum. "Congress provided matching funds. This gave us the $400,000 that was necessary for construction."

Sasaki Associates, of Watertown, Massachusetts, provided the landscaping-architectural design for the garden; this is the firm that also designed the Aboretum's Japanese bonsai collection. Holly Shimizu, the curator of the Herb Garden, says: "The new garden is really three gardens in one; herbalists refer to these as 'rooms.' Plant material masses, changes in elevation and treillage [latticework] are used to separate, yet integrate, the tree rooms -- a knot garden, a rose garden and specialty gardens."

In the Knot Garden, plants are arranged to look like interwoven chains. Although the design is intricate and formal, it is truly simple compared with the original knot gardens of the Renaissance in France and Italy.

The Knot Garden in the National Herb Garden has three interlinking chains, with each chain made of a single plant species -- Japanese holly, dwarf blue cypress and dwarf arborvitae. Crushed brick fills open space between the chains.

Roses are prominently mentioned in every phase of the long history of the culture and use of herbs, long before breeders started developing the hybrids familiar to many gardeners today. They were wild roses, also referred to as Old Roses, species roses or historic roses. These extraordinarily fragrant roses are traditionally included in formal herb gardens. About half the rose collection for the National Herb Garden was obtained in Europe.

The Specialty Garden area features 10 gardens arranged in an oval, each about 40 feet long and 25 feet wide. Each garden reflects a theme: Dioscorides, Early American Dye, American Indian, Modern Botanicals, Culinary, Industry, Fragrance, Oriental and Beverage.

Visitors will probably find more familiar plants in the culinary garden than in any of the other specialty gardens. Included are garlic, thyme, dill, savory, parsley and rosemary.

American Indians used herbs as a source of medicines, dyes, poisons, foods and materials to use in their many crafts. This collection includes plants used by Indians of Eastern North America.

The aboretum grounds are open every day of the year except Christmas. Visiting hours all year round are 8 to 5 Monday through Friday and 10 to 5 Saturday and Sunday.

Q: My wisteria vine is taking over the place. Can it be pruned?

A: The Chinese wisteria, which blooms before the leaves appear, can be pruned without decreasing the number of flowers next spring. Severe pruning causes development of many new shoots, which produce flower buds.

Q: Should I remove the suckers from my sweet corn or let them grow?

A: Let them grow. Research has shown there is nothing to be gained by suckering corn.

Q: I have rooted the top of a pineapple. How should I take care of it?

A: Plant it in a 6" pot, give it full sunlight, at least very good light, 30 to 40 percent humidity, and water when the soil feels dry to your touch.

Q: I want to grow garlic in my garden this year. When is the best time to plant it?

A: Garlic should be planted in the spring about the same time as onions.

Q: I received a gardenia plant for Easter two years ago. It was in full bloom. Since then it has had only a few flowers. What do I need to do to make it bloom?

A: The main problem with trying to grow gardennias in the home is low humidity. These gardenias develop flower buds only when night temperatures are below 65 degrees F. The day temperature is not important so long as it's higher than the night temperature. Buds may form at 70 degrees, but invariably drop off. If the night temperature goes below 60 degrees, plant growth may be retarded.

Q: Do lawn grass clippings make a satisfactory addition to my compost pile?

A: Almost any kind of grass clippings are good for the compost, but it will take a lot of them to yield any volume.

Q: I brought generaniums a month ago. They are in 4" pots and have grown a lot; shouldn't they be transfered into larger pots?

A: One of the first signs of pot binding is a frequent wilting of foliage due to rapid drying of the soil. This is because the plant is too large for the pot it's in. Plants usually need to be changed to pots one or two sizes larger, but fast-growing geraniums may be better off in a 6" to 8" pot.

Q: We have hydrangeas and the flowers are supposed to be blue but are between pink and white. Is there a way to make them blue?

A: The big-leaf hydrangea flowers are blue or pink depending on soil acidity. With plenty of available aluminum in the soil, which occurs with increased soil acidity, the flowers are blue.With neutral or alkaline soil, there is less aluminum and the flowers are pink. Mixing a lot of Canadian peat with the soil will make it more acid.

Q: I see many fields and vacant lots covered with weeds. Shouldn't they be killed? Won't their seed spread all over and be a nuisance?

A: We need to keep weeds out of our gardens and farms, but they help check erosion. Many are attractive and decorative along highways and field edges. Various forms of wild life are dependent on them for food. Birds use the seed and to some extent the foilage.