Saving seed to bring back.

Start with the easy four.

Beans, peas, lettuce and tomatoes are self-pollinating annuals. Each flower pollinates itself, often before it even opens, so the seed carries the same variety you planted. You can save viable seed from a single plant, though five to ten plants keep a variety healthier over the years.

• Beans and peas. Leave the pods on the plant until they are dry and rattle. Pick, shell, and finish drying indoors.
• Lettuce. Let a few plants bolt. Two to three weeks after flowering, the heads turn fluffy and white; shake them into a paper bag every few days.
• Tomatoes. Pick a fully ripe fruit from a healthy plant and ferment the seed (below). Potato-leaf varieties cross more readily, so give them about 50 feet.

Peppers, lima beans and okra are self-pollinating too, but bees visit them often enough to cross them, and hot and sweet peppers cross with each other. Treat them as intermediate: grow one variety, or bag some flowers before they open.

What crosses with what.

Only varieties of the same species cross. A cucumber will not cross with a squash, because they are different species. But zucchini, most summer squash, acorn squash and many pumpkins are all Cucurbita pepo, so grown together they cross, and the seed makes something new.

• The cabbage family is one species, Brassica oleracea: cabbage, kale, collards, broccoli, cauliflower and Brussels sprouts all cross with each other.
• Turnips, Chinese cabbage and broccoli raab are a different species, Brassica rapa. They cross with each other, not with cabbage or kale.
• Beets and Swiss chard are one species and wind-pollinated, so pollen travels a long way.

When you cannot get the distance, separate by time (plant so they flower weeks apart) or by barrier: bag or cage the plants and hand-pollinate. For squash, tape the female flower shut the evening before it opens, pollinate it by hand in the morning, and tape it shut again. The distances in the chart are minimums; bees fly farther where there is little spraying.

The crop chart.

From the Community Seed Network Seed Saving Guide, adapted from the Seed Savers Exchange chart. "Plants to keep a variety" is the population for a gardener saving every few years; seed banks preserving a variety use two to four times as many. Biennials make seed in their second year, so they have to survive a winter in the ground or in storage.

Dry seed and wet seed.

Dry-seeded crops ripen their seed in pods, husks or heads on the plant: beans, peas, lettuce, the cabbage family, onions, carrots, corn, okra and most herbs and flowers. Let it dry on the plant as long as the weather allows, finish drying on a screen or in a paper bag, then thresh by rubbing or stomping and winnow off the chaff in a light breeze.

Wet-seeded crops hold their seed inside fruit: tomatoes, cucumbers, melons, squash and eggplant.

1. Tomatoes and cucumbers: ferment. Squeeze the seeds and gel into a jar with a little water and leave it at room temperature for two to four days, stirring once a day. A skin of mold is normal. Good seed sinks; pour off the pulp and floaters, rinse, and spread the seed on a plate or screen to dry. Fermenting breaks down the gel around each seed and cuts some seed-borne disease.
2. Cucumbers for seed are left on the vine well past eating stage, until they turn yellow or brown.
3. Squash and melons: wash. Scoop, rinse off the flesh, and dry. Winter squash seed improves if the fruit sits for a few weeks after harvest before you cut it.

Is it dry enough?

Seed that is not dry spoils in the jar. The Seed Savers Exchange snap test works for most seed: bend one. Dry seed snaps or shatters. Seed that bends, dents or squishes needs more time. Large seeds like beans and squash take a week or more on a screen in a dry room. Never dry seed in an oven; heat above about 95°F damages it.

Label what you bring back.

A seed library can only share what it can describe. Put each lot in a paper envelope and write:

• the crop and the variety name, as exact as you can make it;
• the year you grew it and the town;
• whether other varieties of the same crop grew nearby, and whether you did anything to isolate it;
• anything you noticed: early, drought-tolerant, heavy yield, disease trouble.

Bring seed only from healthy plants. Some plant diseases travel inside the seed, and home-saved seed has not been through the testing commercial seed gets, which is one more reason to start with the self-pollinating crops and to leave out anything from a plant that looked sick.