If you're planning to grow plants outside, it may soon be time for your students' carefully nurtured seedlings to face the harsh realities of the great outdoors as they're moved to school, home, or community gardens. Transplanting is an excellent opportunity to teach about plants' needs and adaptations and to tie in subjects across the curriculum.
Your students' first challenge will be to decide when to put different plants outdoors. 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 such as tomatoes, peppers, and melons should be transplanted after all danger of frost is past. (You can find out last frost dates in your area from local gardeners, the Cooperative Extension Service, or weather maps.) Students can check seed packets or gardening catalogs to find out about frost tolerance, then count back to decide when to sow each crop indoors so they'll be at the right stage at transplant time.
While investigating planting dates, encourage your students also to determine the frost dates for different areas of the country, and discuss why the dates vary. Or they might research the origins of some of your garden plants and discuss how the cold tolerance or heat preference of different plants relates to where in the world the plant originated.
Have students discuss how they would feel if they were moved from a warm, windless, relatively protected classroom to a harsher outdoor environment. Explore ideas about how they might lessen this "shock" for their seedlings. Hardening off is the scientific term for acclimating seedlings to outdoor conditions. A week or so before transplanting, set seedlings out for a short period (one or two hours) during the daytime. Extend the outside period each day. A couple of days before transplanting, leave the plants out all night as well.
Rather than take our word that hardening off helps plants toughen up, have your students give it a test. Have them conduct an experiment to see firsthand how the health and growth of a "hardened off" plant compares with that of one directly planted outside. Encourage students to imagine and write from a plant's perspective about the challenges (and pleasures?) of being moved from a cozy classroom garden to the cooler, "wild" outdoors.
The best time to transplant seedlings outdoors is on a calm cloudy, or even drizzly day, so that the loss of water from the leaves is reduced. Leaves continue to transpire (give off water) after transplanting, and the damaged roots cannot take in adequate water. You may want to transplant a few seedlings on a sunny day as an experiment so students can observe and compare how the seedlings fare.
Have students use math skills to measure the distance between seedlings (based on seed packet instructions). Young students can use a body measurement such as a hand span to figure spacing. For each transplant, dig a hole a bit larger than the rootball. Mix in compost or rotted manure if available and water the seedlings before transplanting. If you have ample seedlings, you might leave a few unwatered, to allow students to examine the relationship among roots, soil, and water.
When transplanting, have students carefully remove plants from containers, holding the seedlings by their cotyledons (seed leaves), and keeping as much soil around them as possible. Place the plant in the hole at about the same depth it was in the container, and fill the hole with soil. Water seedlings well after transplanting with a dilute manure "tea" or other dilute fertilizer (too strong a solution could hurt tender roots). For the next few days, the newly transplanted seedlings may look droopy or wilted since damaged root hairs take in less water, but they should soon perk up.
To better understand how to treat plants at transplant time, consider conducting Part 2 of "Why Root for Roots?" and "What a System!" in GrowLab: Activities for Growing Minds. The activities will help students understand the roles and needs of roots and stems.