This guide is designed with you in mind, the savvy heirloom seed saving gardener! Of course most of this is according to common knowledge, but below I will list garden varieties commonly grown in the garden each year and how to prevent them from crossing with other varieties. To preserve seed purity and produce "true to type" seeds you'll need to know a little bit about each type of crop! As also as a side note you'll want to take seeds from healthy disease free plants the ones that are productive and vigorous. So here we go.
Beans
Beans both pole and bush seldom cross pollinate with one another since they are considered to be self pollinators. However that isn't to say it won't occur. So as a precaution it is recommended to grow to different colors of beans near one another if you are limited on space and then observe the seeds when ready. The seed will show signs of crossing if you grew to different colors. If you are concerned about crossing separate by 150ft as a rule. Harvest when completely dried; cure them for a week or so by allowing them to further dry out in their pods spread out from one another, and shell, then keep dry for future use.
Lima Beans
Limas can readily cross with other limas, so if you plan on growing to different varieties you must separate by at least one mile. Harvest when completely dried out, shell, and then keep dry for future use.
Runner Beans
Runner beans will not cross with limas or with other common types, however they must be grown ½ to ¾ of a mile from other runner varieties or you could bag the blossoms and shake the bag by hand to pollinate the plant. This is important when wanting to maintain seed purity. Harvest when completely dried out and then shell them to keep for future use.
Beets
Beets will cross pollinate with other beet varieties as well as chard varieties. So with that in mind I would suggest only growing one kind of beet or chard since they require a distance of at least ¾ of a mile. Beets are biennial meaning they must be overwintered in order to produce seed the next year. Some gardeners pull the beets trim the leaves back to 2 inches over the bulb and overwinter them in a root cellar in either damp sawdust or sand. Some gardeners that enjoy warmer winters mulch them and hope they survive the winter temps. Once spring arrives replant them and wait for the seed. Once seed is dry collect and store.
Broccoli
Broccoli will cross pollinate with any member of the Brassica oleracea which includes cauliflower, cabbage, kale, and brussel sprouts so separate by 1 mile. They also require overwintering for seed. So you'll want to preserve the head when overwintering by either heavily mulching the plant or if you have colder winters transplant the plant from the garden to a pot of sand and store over winter in an area between 35 to 40 degrees. Once spring arrives simply place the plant back out into the garden and allow to bolt. Once pods appear and dry out harvest and store seeds.
Brussel Sprouts will cross pollinate with any member of the Brassica oleracea which includes cauliflower, cabbage, kale, and broccoli so separate by 1 mile . It's best to leave these guys in the garden over winter since they are biennial. Simply let some of the heads remain on the stalk. Once spring arrives the heads left should go to seed. Once pods appear and are pick them for seed harvest but make sure not to wait.
Carrots
Carrots will cross pollinate with other carrots or Queen Anne's Lace which is a common weed found in some parts of the country. You'll want to check your area to see whether or not Queen Anne's Lace is grown in your area. If so make sure your carrots are separated by at least ¼ of a mile from Queen Anne's as well as other carrot varieties. Overwinter by mulching and then allow them to seed in spring. Let umbels fully dry and ripen before collecting seeds.
Cabbage
Cabbage will cross pollinate with any member of the Brassica oleracea which includes cauliflower, broccoli, kale, and brussel sprouts so separate by 1 mile . They also require overwintering for seed. So you'll want to preserve the head when overwintering by either heavily mulching the plant or if you have colder winters transplant the plant from the garden to a pot of sand and store over winter in an area between 35 to 40 degrees. Once spring arrives simply place the plant back out into the garden and allow to bolt. Once pods appear and dry out harvest and store seeds.
Cauliflower
Cauliflower will cross pollinate with any member of the Brassica oleracea which includes broccoli, cabbage, kale, and brussel sprouts so separate by 1 mile . They also require overwintering for seed. So you'll want to preserve the head when overwintering by either heavily mulching the plant or if you have colder winters transplant the plant from the garden to a pot of sand and store over winter in an area between 35 to 40 degrees. Once spring arrives simply place the plant back out into the garden and allow to bolt. Once pods appear and dry out harvest and store seeds.
Celery
Celery will cross with other varieties of the celeriac family therefore separate by at least 1 mile. Cut celery down to a stump overwinter. Mulch well and remove mulch in spring for plant to grow. Seeds need to fully dry before harvesting.
Corn
Corn will cross pollinate with other corn varieties and readily. Therefore it is recommended to separate by at least 1 mile. Let seeds dry in stalk but don't tarry in fact make sure to harvest as soon as dry. Also make sure to collect from at least 100 separate corn plants to have a healthy batch of corn next year. Rub corn off cobs when dry and store.
Cucumber
Cucumbers will cross pollinate readily with other cucumber varieties therefore be sure to separate by at least ¼ of a mile or so for seed purity. They should remain on vine well past the eating stage, often the cuke will turn white. Once ready to cut off the vine simply snipe them off with scissors and them set aside to cure for 20 days before harvesting seeds from the center. Simply cut vertically scoop out seeds and pulp into a mason jar of water for fermenting. Then loosely seal the jar with a lid and place in a warm area out of direct sunlight. After 5 to 7 days of fermenting pour out seeds floating on top of the water along with mold and then pour rest of the good seeds, on the bottom, over a fine mess strainer to catch them. Dry well in fine mesh or paper towels but turn often so they don't stick to paper.
Eggplant
Eggplant is a self pollinator but will cross pollinate so with that in mind ¼ of a mile is sufficient for seed saving purposes. Allow them to remain on vine but off the ground. They will take on an a yellow to brown color before ready for harvesting seeds. Scoop the seeds out and over a bowl of water. Good seeds will sink bad seeds will float. Save seeds on bottom of bowl and allow them to dry out like the cucumber seeds.
Kale
Most kales are part of the Brassica oleracea family which includes broccoli, cabbage, kale, and brussel sprouts so separate by 1 mile . Siberian kale is an exception it is not part of this family. Mulch kale to last over winter. Kale will go to seed following year; collect dry seed pods.
Kohlrabi
Kohlrabi is part of the Brassica oleracea family which includes broccoli, cabbage, kale, and brussel sprouts so separate by 1 mile. Mulch kohlrabi much like you would other members of the Brassica family for overwintering. Then remove mulch after winter during spring to allow plants to seed. Plants will go to seed and allow pods to dry fully before gathering for seed. You can also pluck them up and prepare according to broccoli directions above for overwintering and replant next spring.
Leeks
Leeks will not cross with onions, only other leeks so isolate by ¾ of a mile. There is no need to store overwinter simply leave them in the garden until the next year. If you have terribly cold winters you can always mulch them or hill up the dirt around them. Once next year arrives the leeks will produce an umbel which should be ready around the fall; once the umbel is dry pick them and rub them in your hands over a clean surface to release the seeds.
Lettuce
Lettuce is a self pollinator, so you can grow different varieties 50 ft apart. Lettuce is a cool season crop and will begin to bolt, or flower when daytime temps go above 75 degrees. Once they do bolt the lettuce can no longer be eaten as it will be bitter. The plant sends forth a stalk into the air 4 to 5 ft high. Then flowers appear and continue to open for over a month and then the seeds also start coming in. Watch the plant for feathery seeds and then collect them by shaking over a paper bag.
Melons
Melons will not cross with cucumbers or squash but will cross with other types of melons not to be confused with watermelons however. Muskmelons don't cross with watermelons they are from separate families. They need an isolation of at least ¼ of a mile from other melons. Once you are sure they are ripe cut them from the vine and allow to cure for a few days. Once cured cut open scoop out seeds and follow process found under cucumbers for seed saving. You can eat the flesh at this point seeds are normally ready when melon is ripe.
Mustard Green
Mustard greens will cross with other mustard greens within the same species but not the same family. So be sure to check each species before planting near another species of mustard otherwise separate by at least 1 mile. Save pods from plants once they have fully dried.
Okra
Usually self pollinating however can occur within less than 1 mile. So proceed with caution on planting more than one variety. Let pods harden on stalk; they will become woody and then seeds are ready. Simply take them off pod and crack open for getting seeds out for saving.
Onion
Separate onions from other onions by ½ mile. Our preferred method is planting seed to seed meaning planting from seed and then allowing plant to stay in the ground until the following year for seed. You can mulch with hay much like do for garlic. Allow plant to go to seed the following year and watch heads when black seed is seen cut off seed heads and collect seeds. Several cuttings may be required as seeds become ready at different intervals.
Peas; English
Self Pollinating, therefore 50ft is sufficient to prevent crossing. When pods turn brown on plant and seeds rattle inside they are generally ready. Some larger growers often pluck them up with the roots and allow plants to dry out further however this is optional.
Peas; Southern
Cross pollination as with other self pollinates is uncommon among cow peas. However you can space them 50ft apart. Side note: Yardlong is actually in the pea family. Much like English peas or beans collect pods when they are brown and dried out. You also might want to take pods off plants and allow them to cure, by spreading them out indoors or outdoors as long as there is no rain, before shelling to get peas.
Peppers
Peppers are self pollinators, but will cross within the same species much like squash so it's best not to plant two of the same species for more information on the meaning of this read the squash part of the article. Separate by at least 500ft. Once peppers are ripe on stalk and fully dry harvest them cut open and scoop out seeds. Allow to dry for a week or so. Dry method works great.
Pumpkin
Pumpkins are actually from the squash family. So they will cross will different types of squash. There are 4 groups of commonly grown varieties of squash. Cucurbita pepo, Cucurbita Maxima, Cucurbita moschata, Cucurbita mixta. With this in mind any variety from the same species or in other terms with the same last name of the 4 species listed above will cross and must be separated 500ft. For instance you can't grow two pepo varieties together or they will cross. Side note: Be sure to check what type of squash you have in order to determine cross pollination chances. Squash and pumpkins can be seed harvested the same way. Let them stay on the vine until fall and then cut off the vine let sit for a week and then cut them open scoop out flesh and place in water. Wash seeds and place them on a screen rack for drying or paper. Turn often so they are able to fully dry a week or so and then properly them store accordingly.
Radish
Radishes do cross so therefore separate by ¼ mile from other radish varieties. If you are in the North plant from seed in spring and if you are located in the south from seed in fall. Stalk should emerge following spring with little pods. Collect pods when brown and dried on stalk and then shell for seeds.
Rutabaga
Follow seed saving instructions for turnips. Separate from other rutabaga varieties or member of the same species such as Siberian kale by 1 mile.
Spinach
Spinach plants do readily cross so I would recommend at least 1 mile from other spinach plants. When days grow longer and rise in temps your spinach will start to seed. When the plant starts to turn yellow the seed should be ready. Pluck up spinach is they start to bolt to soon, or in other words before the rest of the plants go to seed.
Swiss Chard
Chard will cross pollinate with other swiss chard varieties as well as beets. So with that in mind I would suggest only growing one kind of chard or beet since they require a distance of at least 1 mile. Chard is biennial meaning they must be overwintered in order to produce seed the next year. Some gardeners pull them trim the leaves back to 2 inches over the bulb and overwinter them in a root cellar in either damp sawdust or sand. Some gardeners that enjoy warmer winters mulch them and hope they survive the winter temps. Once spring arrives replant them and wait for the seed. Once seed is dry collect and store.
Squash- Read information for pumpkin and follow accordingly.
Tomatoes
Tomatoes are self pollinating, so separate by 50ft is sufficient. Harvest tomatoes when ripe and you can even let them set for a week or so before saving seeds. Once you are ready to harvest simply cut in half scoop out seeds and pulp into a mason jar of water for fermenting. Then loosely add the lid to the jar and place in a warm area out of direct sunlight. After 5 to 7 days of fermenting pour out seeds floating on top of the water along with mold and then pour rest of the good seeds, on the bottom, over a fine mess strainer to catch them. Dry well in fine mesh or paper towels but turn often so they don't stick to paper.
Turnips
Separate turnips from other plants in the Brassica Rapa family. Plants are biennial so will require overwintering with mulch or trim leaves to 2 inches above the bulb and store them in a root cellar in either damp sawdust or sand. Replant in spring and wait for them to flower; when pods dry out they are ready.
Watermelon
Watermelons will cross will with other watermelons so isolate up to a ¼ mile or so. Once you are sure they are ripe cut them from the vine and cut open scoop out seeds and follow process found under cucumbers for seed saving. You can eat the flesh at this point seeds are normally ready when melon is ripe.