When grapevines are properly pruned every year they usually produce a better harvest. The main purpose of pruning is to prevent the vine from bearing too many grapes. Too big a crop causes delayed ripening, the grapes are not as large and sweet as they could be and there is poor fruit bud formation for the following year.

The best time to prune is late winter before the vine starts to break dormancy. If it is done after the sap starts to flow, heavy bleeding may result. The bleeding does little if any harm and specialists say it is better to prune late and take the bleeding than not to prune at all.

An arm is an old stem growing from the truck. Attached to the arms are canes. On canes that developed last year there are a number of joints or nodes and at each there is a bud. When the weather is right in the spring these buds open and shoots start to grow. Each of these shoots usually develops two to four blossom clusters in addition to leaves. A few weeks later the blossoms open and fruit forms. The fruit ripens in late summer.

The idea in pruning is to remove all but four to six canes for fruiting and about four to six for renewal (provide canes for next year).

The canes for this year sould be four to six feet long and about the thickness of a lead pencil or slightly larger. One cane per arm is the best rule. These canes for fruiting should be shortened so that eight to 15 buds remain on each, a total of 32 to 72 buds remaining on the vine.

If the vine has not been particularly vigorous and if it bore too many grapes last year, it is better to leave only four canes with eight buds on each.

This may sound severe but it is necessary to get high quality fruit.

The base of the four to six canes for renewal should be as close to the trunk as possible. A cane growing on the trunk is good for this purpose. Shorten these canes so that one or two buds is left on each. A strong shoot will grow from one of these buds and this shoot will provide a fruiting cane for next year.

Grapes are different from most fruits in that they must develop all of their flavor while still on the vine. The best way to determine when they are fully ripe is to pick a few and taste them for sweetness. Q: My forsythia bushes are badly in need of pruning. When is the best time and do you have any suggestions on how to do it? A: Forsythia usually needs to be pruned every year after you have had it three or four years. This is also true of deutzia, mock-orange, flowering quyince, bridal wreath spirea and weigelia. The best time is later winter but it can be done after they finish blooming. The canes should be cut off at the ground level and new ones will grow to take their place. The bark of the new canes is smooth and bright in color. As they age the bark becomes duller in color and there are fewer buds. These are the canes that should be removed. Q: There are poison ivy plants growing at the base of our lawn. Is there a fool-proof way to get rid of them? A: After growth starts in the spring, spray the foilage with Amitrole, sold under the trade names of Poison Ivy Killer, Weedazol, ATA. Keep it off the plants you don't want to hurt, including the grass. SPRINGTIME IN BALTIMORE -- Spring's already arrived inside the Baltimore Convention Center where daffodils and tulips, azaleas and cherry blossoms, sculptured gardens and waterfalls are on display in the annual Flower and Garden Show through Sunday. Visitors can quiz horticulturists and landscapers and see demonstrations from flower arranging to vegetable gardening. Admission is $4, $2 for children age five to 12. POTOMAC ROSES -- The Potomac Rose Society will hold its monthly meeting Thursday at 8 in the Town Hall Meeting Room, lower level, Tysons Corner Shopping Center. Landscape planner Tom Derickson will talk about minimal maintenance landscaping