I haven't made a study of it, but it seems like there ia an optimum spacing. Once they get more crowded, they may produce about the same amount, in the same space, as when grown with more space, but crowding doesn't increase the harvest. The optimum space will vary a bit, depending on the variety. Some beans are way more vigorous than others.

Optimal spacing can vary, since many variables can impact that decision. I remember a good thread discussed this in the past, where I posted photos of huge solitary pole bean plants.

Close spacing is good if you want a large crop quickly; but this generally comes at the expense of a much-reduced harvest period. If planting late in the season, however, close spacing might offer the best chance for a good harvest. Such spacing also might be good for drier areas, where it would help to conserve moisture & reduce soil temperature... however, in cool and/or damp climates, such close spacing often results in increased susceptibility to disease.

The amount of seed available can be be an influence. If you are starting with only a small seed quantity (as is often the case with trades) then wider spacing would allow the highest yield per plant... 8-16 ounces of dry seed from a single pole bean is possible. If seed is plentiful, obviously spacing is not as important.

And as Macmex pointed out, some varieties with sparse foliage can be spaced more closely; I find that the pole bean Fortex falls in that category. Denser, more heavily-branched varieties (such as the bush bean Woods Mountain Crazy Bean) will require more space. Limas in general tend to have higher vigor than common beans, and can be spaced more widely.

In my experience, wide spacing produces healthier plants, a more sustained yield, and larger, more vigorous seed. As a seed saver, that larger healthier seed is a deal maker. And in my climate, I often find the need to fall back on bean transplants, when direct seeding fails due to weather. In such cases, getting the best performance from a limited amount of transplants requires wide spacing.

The standard spacings that I use are just starting points, which can be adjusted for each variety after observations of their response.

• bush beans: 6-8"
• pole beans: 6" (or 2 @ 12") for snaps, 12" for dry seed
• runner beans: 8-12"
• yardlong beans: 6-8" for pods, 12-15" for dry seed
• bush limas: 12"
• pole limas: 18-24"

It should be noted that these are spacings which work in my short seasons. Where seasons are longer, and growth more vigorous, plants might benefit from even wider spacing. I found this difference to be most noticeable when I grew pole limas in San Diego, where they covered the trellis even at 36" spacing. Pole beans continue to astonish me, in their ability to branch out & overtake any adjacent open space.

Very interesting info, thanks, I posted a similar question on the veggie forum and was directed here. On the list above it says : pole limas: 18-24"

Is that the pole spacing or the single bean plant spacing, one per pole?

btw, I always grow bush beans and generally space them 2" apart, double rowed 1 ft apart within a wide row (2 rows in one). Before blooming I stake up twine on the outside rows to support the plants during storms. An irrigation hose goes down the middle.