I tried using isolation cages for bush beans, the same ones I use to get pure seed from peppers. Unfortunately, the conditions under which peppers thrive (heat & humidity) promote disease in beans... so I was forced to discontinue the experiment, to save the plants. I used floating row cover; a screen cage might allow greater air flow, and avoid the disease problems.

For common beans, since they self-pollinate, all that is needed to produce pure seed is a little distance. The distance necessary is a topic of considerable disagreement; search GW for "bean cross", and you will find many discussions of the issue. In general, if you grow two beans on opposite sides of your yard, the seed should be reasonably pure - for your own use. Grow one in pots in your front yard as well! But if you are preserving an irreplaceable heirloom, I would use a greater distance, or grow only that bean for one season.

My yard is just over an acre, and I grow three beans for seed each year at home, 100+ feet apart. In my main plot (on a friend's property) I grow several others in a 100-foot square; but I trellis other plants between them, and plant flowers throughout to act as "cleaning stations" for pollinators. So far, no crosses.

But I can tell you that I grow many beans obtained through swaps, and I have yet to go a year without seeing crosses... so they do occur. Sometimes they are so bad that even after roguing out the plants with obvious differences (which is a good practice) the remaining seed is still impure, and canÂt be saved.

I nearly lost a rare variety that was sent to me this year; only _one plant_ was true-to-type. It may take me several years to clean it up to the point where I can re-offer it to others.

Limas & runner beans are more difficult than common beans. Bees are strongly attracted to the blossoms, so if you grow more than one, they _will_ cross... and that also includes any of your neighbors within at least 1/4 mile. I have never tried caging them; but it is my observation that while they are both supposedly self-fertile, they seldom self-pollinate without assistance from insects. You might be able to grow them under a cage, if you "trip" the flowers to simulate an insect visit.

Zeedman, interesting that you mention 'tripping' the flowers. Found two pages discussing isolating beans in cages to prevent cross-pollination, one of them is a PDF that discusses tripping.

My apologies if these have already been posted and discussed here..

http://gears.tucson.ars.ag.gov/book/chap4/lima.html
http://crop.scijournals.org/cgi/reprint/39/2/428.pdf

One other aspect, HappyDay. Exactly how many plants is "a few?"

In order to preserve the genetic characteristics of the population, beans should be saved from at least 20 plants.

True, there are some instances where we can't do that, particularly with a rare variety that we only have a few seeds of. But anytime we save from fewer than 20 we risk losing some of the genetic material that determines normal variations within the population; and can expect loss of genetic vigor over time.

That last takes longer with beans, because they are so in-bred to begin with. But, even so, they do run out if you save seed from too few plants.