Know-how can make a big contribution to successful gardening. Learning by trial and error is often expensive and almost always frustrating. For example, if you make a mistake early in the season with your tomatoes, you may have to wait until the following year to get a good crop.

An excellent new book covering gardening basics eliminates a lot of the guesswork. Another good new one tells how to grow plants in water both outdoors and in the home.

"The Gardener's Hint Book," by Dr. Charles L. Wilson (Jonathan David Publishers, 480 pages, well illustrated, $12.95). Wilson, an award-winning specialist in plant diseases, was research leader of the USDA Shade Tree and Ornamental Plants Laboratory in Delaware, Ohio, for seven years. He now does research on tree diseases at Ohio Agricultural Research and Development Center, Wooster, and for several years has been in the forefront of efforts to save the American elm from extinction.

In his book, he outlines the basic principles involved in growing vegetables, flowers, fruits, trees, shrubs and house plants. The book offers a wealth of hints and suggestions: When fencing a garden to keep out animals, remember that some animals are diggers and can go under normal fencing. Bury garden fencing at least six inches to keep diggers out.

Recommendations are sometimes made that grass seed be sown on top of snow. This method has its shortcomings. When the snow melts during a warm spell, some of the seed washes away, and birds eat the rest.

Flower beds can become bothersome. To convert them to areas requiring less care, replace the flower bed with a ground cover. Since the soil is already prepared and the bed has been designed to fit into the landscape, little bother is involved.

Before purchasing trees, remember that deciduous trees (those that lose their leaves in the fall) should be planted on the south side of the house. They will provide shade in the summer and let the sun in during the winter.

Flowers of different colors are often difficult to harmonize in a flower bed. Interspersing plants with gray foliage (such as dusty miller) is an excellent way to blend them. White, which some people consider a blender, actually separates the colors and makes them more distinct.

Plastic bags placed over corn ears prevent birds and raccoons from harvesting the corn before you do. Just before the ears reach maturity, cover each with a plastic freezer bag and tie it to the base of the ear.

"Water Gardening Indoors and Out," by Reginald Dutta (Crown Publishers, 128 pages, well illustrated, $10). Dutta is managing director of London's oldest fish tank specialists. His fame as a "fish doctor," his trend-setting original fish-tank designs and their interior aquascaping have gathered him an international clientele. His book describes the methods and equipment used in growing things in water, well within amateur capacity, and with a series of displayes, photographed in color and black and white, demonstrates how to get the most out of the hobby.

There is a section specially devoted to hydroponics (cultivating plants in water without soil), an approach offering infinite visual possibilities, ideal for the apartment dweller.

"The aim of this book," says the author, "is to focus on the plants: their wonderful variety, the uplift they can give you in a gentle, yet very pervading way; yet ever ready to welcome you back, to show you their new growths, to sway in response to your smile; they will soothe your cares, ease your hurry, your rush, your tension. They will give you a quietly sustaining strength of green harmony that perhaps you do not fully expect."