Once your students have created visions and plans for bountiful outdoor gardens, the next step is bringing them to life. If you're in an area with a short growing season and/or you want to harvest certain crops before school is out for the summer, you can get a jump on the season by raising your own seedlings. You may also want to start seedlings indoors if you've planned a community beautification project, spring plant sale, or if you plan to bring plants to senior centers, homeless shelters, or send them home as gifts. Raising seedlings in the classroom offers a good opportunity to practice reading and language skills (via seed packet planting instructions), math skills (e.g., using seed packet information to determine when to plant), and science process skills (predicting germination times or inferring why seedlings are leggy).
Your students' first challenge will be to determine when they'll want to have seedlings ready to give away, sell, or plant outdoors. To develop a planting calendar, they'll need to know:
Students can check seed packets or gardening catalogs to find out about frost tolerance, then count back to decide when to plant each crop, and develop a planting calendar. While investigating planting dates, students may also want to find last frost dates for different areas of the country, and discuss why the dates vary. Or they might research the origins of some garden plants and discuss how their temperature preferences may relate to where in the world the plants originated.
Containers. Many types of containers will work, as long as they're at least 2 to 3 inches deep and have drainage holes. To save space with seedlings that are easy to transplant, you can sow seeds closely in shallow containers (try recycled containers like milk cartons), then transplant them later to larger individual containers or sections.
Soil. Use home-mixed or premixed soilless potting mix for starting seedlings, since it is light, holds water, and is weed-free and sterile. Although your students may want to experiment and compare soilless mix with real soil (and should be encouraged to do so!), garden soil does tend to harbor weed seeds and fungus, and is often too heavy for tender seedlings.
Light. Although you can grow seedlings on windowsills that get plenty of light (south-facing windows are best), these seedlings tend to be "leggy," and will generally grow better under fluorescent lights. Seedlings grow best with 14 to 16 hours of light a day, much more than windows can supply in late winter. To prevent stretched, leggy stems, the lights should be kept within a few inches of the top leaves.
Planting and Maintenance
Planting. Before planting, wet the soil mix thoroughly. Fill containers, then tap them to settle the soil without packing it tightly. You can either sow seeds that transplant easily closely in rows spaced an inch apart, or scatter before covering them with soil. A good rule of thumb is to plant seeds about two to three times as deep as they are wide. A few types of seeds either require light to germinate (check the seed package) or are so small that you should press them gently into the top of the soil without covering them. Be sure to keep germinating seeds moist by covering the containers with waxed paper or gently sprinkling them regularly so they don't dry out.
After the first true leaves appear (after the cotyledons), gently tease out closely planted seedlings with a pencil point or popsicle stick and transplant them to containers where they'll have more space. Try to lift seedlings by their cotyledons rather than by their lifeline: the stem.
Start more sensitive crops like squash or melons in a small cup or container where they'll remain. Plant three or four seeds, then cut all but one see after the true leaves appear. Plan to put them in the garden after danger of frost, to avoid disturbing the roots.
Tending Seedlings. It's best to water seedlings when they need it rather than on a regular schedule. Have students test soil moisture with a finger, and water only when the top 1/2 inch of soil is dry. You can begin fertilizing seedlings once their first true leaves have formed, but be careful not to overdo it. The right amount of fertilizer will help keep your seedlings looking dark green (rather than pale yellow), but too much can be harmful. A good rule of thumb is to fertilize with half the recommended dose once every 10 to 14 days. Students may want to experiment to discover for themselves the consequences of too much of a good thing!
Hardening Off. Before you move seedlings outdoors, "harden" them off to get them accustomed to harsher outdoor conditions. Do this by setting them outside for progressively longer periods each day, starting with a few hours and increasing to a full day over the course of a week or so. Your students may want to experiment to see how the health and growth of a hardened-off plant compares with one planted directly outside.
What to Transplant, and When. The best candidates for an early indoor start are those that tolerate root disturbance and that benefit from a jump on the season. "Cool weather crops," like broccoli, cauliflower, and cabbage can be set out up to a month before the last danger of frost in your area. "Warm weather crops," like tomatoes, peppers, and melons should be transplanted after all danger of frost is past in your area. Flowers' planting requirements vary so check the seed packets. Root crops such as carrots and beets don't transplant well, nor do corn, beans, peas, squash, and cukes. Melons have tender root systems, but take a long time to mature, so can be started in individual containers and transplanted while still small, after the danger of frost passes.