Once your carrots have been dug and tomatoes harvested, discuss with students how you might put the garden "to bed" for the season (unless you can garden through the winter in your climate).
It's important to remove garden plant debris so it won't harbor pests and diseases that could re-emerge the next year. But if nature has its way, the bare ground left behind will have its nutrients leached out, loose topsoil blown or washed away, and soil covered with a mat of weeds. One way to counter this scenario and to enhance lessons on root function, nutrient recycling, and decomposition, is to plant a "cover crop." A good cover crop grows fast and competes with weeds, pulls up nutrients from deep in the soil, and, once it's tilled or dug in, decomposes to add new nutrient wealth and "tilth" to the soil. (Hence the designation "green manures.")
Two of the most important groups of cover crops are grasses and legumes. Grasses (like wheat, rye, and oats) are hailed for their ability to grow fast and use their extensive root systems to "catch" soil nutrients before they leach away. Legumes (like alfalfa, fava beans, and clover) can actually "fix" nitrogen from the soil and make it available to plants.
The types of cover crops you choose to plant in the fall depends on your region of the country and your gardening schedule. In the Northeast, for instance, annual ryegrass or oats are choice cover crops, since they grow quickly but will die over the winter, so they won't become weedy in the spring. Winter rye can be started as late as mid-October (and will grow again in spring), so it would be a choice if a garden is "occupied" until late in the season. (But be sure to till it in the spring before it goes to seed!) Southern gardeners who can grow year-round might choose to rotate a cover crop into a garden section, to improve the soil and give it a rest. To learn about recommended cover crops and planting times for your area, have students contact your local Cooperative Extension Service or area nurseries.
Consider how you can use the concept of cover cropping to spark investigative learning. Students might explore what qualities and adaptations make grasses and legumes ideal cover crops, compare the effects of different types of cover crops on soil, or compare garden plots raised with and without preceding cover crops, for instance.