Learning to read between the lines of garden advertisements

You place a $25 order with a mail-order nursery to get a pink dogwood tree listed in an ad as a ''free bonus.'' The photograph shows a 20-year-old tree, but the tree you receive arrives in a small envelope.

You pay $100 or more to a landscaping firm to service your lawn with fertilizer, peat moss, and weed control but soon discover your lawn is no better than your neighbor's lawn, which did not have the ''treatment.'' You further discover that the firm used a combination of straw, sawdust, wood shavings, and traces of horse manure that eventually will kill your lawn.

If you have ever been a victim of reading what was not in an advertisement or thinking you were going to get something for nothing, then join the hundreds of other consumers who also believed they were getting a bargain.

''Advertising omissions and exaggerated claims appear daily,'' says Hugh Exnicios, director of the Council for Truth in Advertising.

''Not all (garden) advertisers are evil with intent on fooling you. Most are legitimate businessmen conducting honest transactions. But some advertisers have developed their skills so well that most ads seem to offer worthwhile and tremendous savings. The language, drawings, and photos in the ad make the garden product irresistible.''

What can you do to make sure when you buy a mail-order garden product or contract for a gardening service that you are receiving fair and legitimate treatment?

Consumer organizations all warn people buying garden products: Invest your time researching and checking before you invest your money! These groups include the Council on Consumer Information (CCI), Stanley Hall, University of Missouri, Columbia, Mo. 65201; the Center for Consumer Affairs, 929 North Sixth Street, Milwaukee, Wis. 53203; and the Council for Truth in Advertising, 3720 Clearview Parkway, Metairie, La. 70002.

* Fertilizer. Gardeners should know that a complete fertilizer marked 1-1-1 on the bag - the ratio of nitrogen, phosphorus, and potassium - contains only one-tenth the nutrients in a fertilizer marked 10-10-10. Avoid unmarked bags and neighborhood truckloads of ''humus'' and beware of itinerant landscapers selling fertilizer.

* Peat moss. Some advertisements say peat moss will make the soil more fertile. The opposite could happen to deplete the soil if you spread large amounts and don't add nitrogen. Peat moss improves the water-holding quality of the soil. Soil with a high organic content or a shallow water table needs little or no peat moss.

Peat moss sells by the weight and can have from 5 to 10 percent moisture.

* Grass seed. Ten-pound bags of grass seed advertised for a dollar or two will usually sprout quick-growing coarse grasses lasting a year. You can save time and money in the long run if you buy a pound or less of smaller, more expensive seed that sows an area much larger than the bigger cheap seed. An ounce of bluegrass, for example, contains 135,000 seeds, while an ounce of alta tall fescue has only 14,000 seeds. The seed bag should give an analysis of the grasses. Buy bags with a 10 percent or lower mixture of temporary or annual seeds.

* Plants. Avoid mail-ordering plants without the full botanical names. You should check the plant's characteristics in a standard reference work. Watch out for beautiful bonus plants that could be finger-high cuttings and collected wild species developed for home growing.

Know the plant grade in accordance with the American Standard of Nursery Stock. The American Association of Nurserymen grades nursery plants. Roses have a grade of 1, 11/2, 2, culls, and rejects.

Culls (inferior quality), half-matured seedlings, weedlike and diseased plants, bare-root evergreens, unlabeled specials, one plant of a kind when pollination requires two, plants grown in warmer climates or sold in the wrong season - all could be the stock from bargain-basement inventories.

* Bulbs. Mail-ordered bulbs at a ''fantastically low price'' often are the size of almonds and won't bloom for three or four years. Nursery growers advertise bulbs by diameter or circumference. Know what measurement the ad uses.

Consumer agencies caution home gardeners to consult a good gardening encyclopedia before mail-ordering from a catalog or circular. Or deal with an established local nursery with graded plant stocks and guaranteed replacements.