AIRVILLE, Pa. - It's the time of year when gardeners get an itch to start playing in the dirt again.

Slush may still line the streets and a chill may linger in the air, but they're already daydreaming about plant layouts and strolling through garden centers to check out the newest varieties of seeds.

Although it's still too early to plant a garden, it's the perfect time to start some stellar seedlings to transplant later.

If you've never started seedlings, experts suggest you let the four basics be your guide: light, warmth, moisture and soil.

Young plants need plenty of light, warmth and moisture to germinate and grow, but they also need the right kind of soil. "You don't want to use soil out of your garden because it's too full of pathogens and seeds and stuff," said Frank Sommer, 73, of Chanceford Township.

Sommer has been a Penn State master gardener since 1989 and has been growing plants since he was about 8 years old, so he's quite an expert on how to get the leafy living things off to a great start.

By mid-March, most vegetables and flowers can be started from seed indoors. Sommer said vegetables such as carrots, beans, potatoes, onions and lima beans don't transplant well, so it's a good idea to wait and sow those where they will grow in an outdoor garden.

The best advice he can give to beginners is: "Read the seed packet."

Punch holes in the bottom of small containers, or use seed-starting trays. Bonnie Oberlander, garden center manager at Shiloh Nurseries in Emigsville, said if you're punching holes, a good guideline is to make four holes for every 8 inches of container for proper drainage.Fill trays or containers with seed-starting mixture.Scatter seeds evenly over the surface of the seed-starting mixture according to the instructions on the seed packet. Do not crowd the seeds.Cover the seeds with a shallow layer of the mixture if necessary (see seed packet instructions). In general, seeds are covered to a depth twice the diameter of the seed.Sommer said most seed-starting mixtures come already moist, but if they aren't, sit the trays in a flat saucer of water after the seeds are planted and let the seed-starting mix absorb water through the holes in the bottom of the trays. Once the surface of the seed-starting mix starts to change color, it has absorbed enough water."You shouldn't let the surface dry out, but you don't want it to get so wet that you're going to drown the plants either," he said. "You need an educated finger, really."Label the pot with the plant name and date of planting.Invert a clear plastic bag over the tray or container and place it in a windowsill facing south or under fluorescent lights. The bag will maintain the proper humidity and moisture level to make the seeds sprout.Keep an eye on the trays. Once they sprout, remove the plastic bags and begin to water as needed.Once the initial "seed leaves" disappear and the first few real leaves appear, it's time to transplant the sprouts to larger containers.If using seed-starting trays, turn each square section upside down. With the stem of the plant between your fingers, tap the edge of the container on something until the plants drop out, or pop them out by pushing up on the bottom of the container. If using another type of container, dig up each sprout.Plant the sprouts in containers about 2 inches across, filled with potting soil. They should be planted just slightly deeper than they were in the seed-starting containers.Sommer said when handling seedlings, it's a good idea to hold them by one of the leaves rather than by the stem. "If you break a leaf, the plant keeps on growing, but if you break the stem, the plant dies," he said.After May 1, harden off the plants by placing them outdoors in a location away from strong sun and wind for a couple of hours each day. Be sure to bring them back inside overnight.Each day, leave the plants outdoors just a bit longer than the day before. Repeat for three or four days before transferring them to an outdoor garden."If you put them out all of the sudden without hardening them off, they'll probably die," Sommer said.Transplant the tiny plants to the garden according to instructions on the seed packet.Water the transplanted plants to settle the soil and get rid of air pockets.Fertilize with a powder fertilizer that has the middle number as the highest, since that will make the root systems grow.

Common mistakes, according to Sommer and Oberlander, include:Sowing seeds too deeply.Watering seeds by pouring water on top of them rather than letting the trays soak up water from underneath.Watering too much.Discarding seed packets after sowing, or not following the instructions on them.Transplanting seedlings outdoors without watering them.Transplanting seedlings directly from indoors to outdoors, without placing them outside for a few hours each day to let them adjust first.Packing soil or seed starting mix too tightly around a seed or seedling.

Sommer's tips for starting seedlings:Handle leaves, not stems, of seedlings.Use warm rather than cold water to water seedlings. Warm water is more easily absorbed.Place seeds under fluorescent lights during germination and growing.Transplant sprouted seedlings to larger pots earlier rather than later to avoid a tangle of roots and leaves.Transplant seedlings outdoors on cloudy rather than sunny days, or in the evening, so they have time to recover from the disruption before being hit with hot sun.