Whether you've started your own plants from seeds or plan to buy them from a garden store, you can get more mileage this summer by giving them extra care this spring.
Most home gardeners like to buy flowering plants that are showing color so they can see what they are buying. One disadvantage of this practice, however, is that blooming plants are more difficult to transplant unless they are in individual containers.
If they are in bedding-plant boxes that accommodate six to 12 plants, the roots will be torn when they are separated. The bigger the plant, the more injury and the longer it takes for the plant to recover from the shock.
Many growers now are using plastic containers that are divided into cells to keep the plants' roots separated so there is little or no transplanting shock as long as the plant roots do not get too bound up from being confined too long.
Plants such as impatiens withstand a lot of root crowding as long as they are in separate compartments. Petunias, on the other hand, get hard and tough-stemmed; any tall stems should be pinched back at least by half.
All plants, including impatiens, will benefit from pinching them back if they have grown ''top-heavy'' in their containers, and this can be done before planting out.
Pinching is a stimulating process, and it will cause small buds or side shoots to ''break out.'' This makes the plant more compact.
If you have a choice, buy the tender, shorter plants. They'll take off faster after transplanting. An exception is the tomato, which can be planted deeply, with just the tip sticking out; the roots will form all up and down the stem.
It's a good idea to ''harden,'' or acclimatize, plants before transplanting them outdoors, especially if the weather is much cooler than they've been used to indoors. The stems and leaves will be tender unless you start giving them a lower temperature and a little less water to toughen them up.
Home gardeners who have a cold frame can move plants to this structure, or they can move them to a sheltered outdoor spot where the temperature doesn't go below 50 degrees F. and leave them there for a few hours each day.
If a cold frame is used, watch the night temperatures. You can throw a blanket over it if the temperature starts to dip.
When setting plants outdoors, try to do the job after sundown or on a cloudy day. An hour or two before transplanting, water your plants, whether in trays, pots, or ''paks.'' This allows them to fill up with moisture, an insurance against transplanting wilt.
When removing the plants from the container, try to keep as much of the soil as possible around the roots. If the plants are in market ''paks,'' you might want to cut them apart in blocks, just as you'd cut brownie squares.
If the roots are not too matted, they can usually be nudged apart, without tearing too many.
Plants should be set slightly deeper than they grew in the container. If the plant is in an individual pot, try to knock it out intact by inverting it and rapping the rim of the pot sharply (with upward strokes) with a trowel handle. A quick tap on the bottom should dislodge any stubborn roots.
Some gardeners like to put fertilizer in the bottom of the hole before setting in the transplant. If it is dry fertilizer, we suggest a sprinkling of soil over the top before setting in the plant. It should then be watered before firming all the soil around it.
If you use liquid fertilizer, you can pour it on as you firm the soil around the roots. Firm the soil gently; don't tromp it - and don't mound it around the stem.
Leave a slight depression (like a saucer) to catch water.
If cutworms are a problem, slip some aluminum foil around the stem. Or you can use wax paper, large split soda straws, or the cardboard roller from bathroom tissue. The cutworms that attack newly planted flowers or vegetables won't climb up the barrier, and thus are foiled.
Also, have at hand some gallon plastic jugs with the bottoms cut out so they can be placed over the plants if it turns cold; or if hot, dry winds beat against the plants.
Containers made of peatmoss are popular with home gardeners as well as commercial growers.
Jiffy-7 Peat Pellets and Jiffy-9 Plant Starter Pellets are so named because they expand to 7 or 9 times their size when watered, as do chocolate wafers. Seeds are planted in the small holes in the center and are surrounded by a nylon mesh.
There is one precaution: When the plants are ready to be transplanted, sometimes the mesh doesn't break down and the roots will have a difficult time getting through it during the growing season. Using scissors or a one-sided razor blade, make slashes in the netting at transplanting time so roots can get through.
Another precaution: When setting out individual peat pots (made of compressed peat), make sure the top of the pot is an inch or so below the soil level. If the peat pot is exposed, it acts as a wick and the transplant will become dehydrated as water is drawn up from plant roots and evaporated by sun and wind.
If this happens, the pot will return to its dry state, stopping the plant's growth by becoming impervious to water.
Home gardeners who start their own plants from seed often have trouble knowing when to plant the seed. As a result, seedlings often become too tall by planting-out time.
Try to time your the sowing of seeds according to the date you'll be able to plant them outdoors, in other words, at the last normal frost date. In our area of upstate New York, for example, the traditional outdoor-planting date is the last week in May. As a result, we sow tomato seeds about the 8th or 10th of April, since tomatoes need about six weeks from seed to setting-out time.
Seed packets usually tell how many weeks are needed, and many catalogs also have this information.
Gardeners can ask seasoned growers when it's safe to set out the tender plants. Frost dates can vary within only a few miles because of differing elevations. This is why commercial bedding-plant growers must have plants ready over a period of four to six weeks in their general area.
The secret is not to ''rush the season,'' because you gain nothing by setting tender plants out before the soil and air temperatures are warm enough to produce growth. A frost could wipe out your entire garden.
There are commercial items available to protect plants from frost, such as hot caps and other plastic products. In fact, you can make your own caps from plastic jugs or inverted glass jars, but make sure you remove or ventilate them on hot days. Mulch (especially leaf mold) around plants also absorbs and holds heat. Black plastic around melons has long been used for this purpose.