Well, I use 8 foot t posts to support pole beans meaning about 6.5 feet are above ground and usable.

Even with this I end up with beans massing at the top.

There are alternate ways of course, but if you are looking to grow straight up or even slightly angled such as with the tee-pee method it is unlikely vining beans will stop at a height you can still reach.

In such a case you have a couple options. Allow the beans to overflow and do their thing. Just accept that you must harvest the beans in that top, tangled mess.

Alternately you could 'top' the beans which is as simple a running a hedge trimmer along the top of your trellis and shearing off anything growing over the top.

Perhaps others have better ideas. I don't really know how tall a pole bean would grow if allowed unlimited height, but I suspect that in a good year multi level scaffolding would be required to harvest ;-)

Pole beans typically grow from 8 to as much as 15 feet. If you build a trellis that high, however, you won't be able to harvest them.

I have permanent trellises. The frames are 1 1/2 inch PVC, and the actual trellises are jute, which get's replace every year.

The way I designed them, the side poles are adjustible, so that I can raise it for a particularly rampant variety. But, in general, I leave them 8 feet, which is as high as I can comfortably reach.

Pole beans will overtop any reasonably sized trellis. This is not a real problem so long as you remember that the whole thing will be top-heavy, and you have to build it strong enough to support that imbalanced weight.

I have always cut them off once they exceed the trellis height for fear I would miss a bunch in the mass at the top and production would slow.

Is slowed production a myth with poles?

This runs a little long, so I hope you will bear with me...

I suppose my biggest question is... a 7' trellis for soybeans?? I grow more soybeans than most, as part of a preservation effort (29 varieties this year). While some can lodge & tangle, very few will climb. A loop of string wrapped around the plants, between 2 poles, should provide adequate support for even the tallest variety. Most commercial "edamame" types are just over 2' high, and are self-supporting, unless grown in shade.

As for pole beans, I agree with Gardenlad; given optimal conditions, 8-15' vines are not unreasonable. I once raised a 15' pole in the middle of some "King of the Garden" limas, just to see what they would do - they climbed the entire length, and were still reaching for the sky. Of course, I couldn't pick them... (LOL)

I use roughly the same trellising method as Username5, T-posts with a hand-made twine mesh between them. You can use fencing or construction remesh in place of the twine; but some beans (such as yardlongs) prefer natural materials. _Rusted_ fencing (or rusted construction remesh) works better than zinc-coated or plastic.

Regardless of your pole or trellis, the beans could outgrow it. So you are left with several options:
(1) Trim the vines above the top. This may stimulate new branches, but they will also be mostly at the top, so it will be a short reprieve. And for some varieties (like runner beans), that main runner is the most productive.
(2) Let the runners grow long, train them further down onto the trellis, and let them climb up again (my preferred method). Takes some time, though.
(3) Space your plants farther apart & then do nothing; you will have much more manageable top growth.
(4) Crowd your plants slightly to stunt the runners. Each variety responds to this differently, and not always successfully. But with a little experimentation, you should be able to find a spacing that closely matches your trellis. This spacing should also give the highest yield. For example, when I crowd yardlong beans, they revert to a half-runner habit, more like cowpeas. The downside: crowding restricts airflow, and with moist conditions, can promote disease.

The relationship between pole bean production & selective pruning deserves greater discussion in a separate topic.

Username5, I have observed slowed production in pole beans, and for the most part agree with Gardenlad. He makes a good point about biomass triggering. But I have also observed periods of deformed pods following periods of heavy yield; removing all pods (I even remove any remaining blossoms) and tip pruning stimulates new growth, and a quick return to heavy high-quality production. This involves only a modest reduction in biomass.

I have read that the developing seeds release hormones into the plant that push it further into its reproductive cycle, and inhibit new growth. The greater the number of seeds, the greater the growth reduction. When a particular number is reached (and that number will vary with spacing, location, weather, and even year-to-year), flowers & young pods will abort, regardless of fertilization. Absent those hormones (when pods are removed before drying), the plant returns to a growth cycle, and puts out new branches & blossoms. I grow beans that are harvested in the green-shell stage, and can confirm this behavior in beans first-hand.

Perhaps biomass & hormones are just two ways of discussing the same process.

The question should perhaps be (for snap beans), at what point does the vine stop bearing _quality_ pods?

There seems to be _considerable_ variation between P. vulgaris varieties in their bearing cycles. Some will resprout on their own, as Gardenlad mentioned, fairly quickly. Some tend to bear only once; "Goldmarie" performs this way for me.

And some will continue to bear only sub-standard pods for a time, unless forced immediately into regrowth; I grew "Pole 191" for many years, and it had that tendency. With my short seasons, I need the vines to return to full production (or as I say, "rebound") as quickly as possible. Most of the snap beans that I have grown seem to benefit, to a greater or lesser degree, from the pruning methods described. Perhaps they are only necessary in short-season areas, or with a small number of plants; but they proved helpful even when I lived in SoCal.

But environment seems to also play a role in pole bean slowdown. I have grown "Fortex" for many years. While it seems to have a more sustained yield than the others I have grown, it does display some periods of slow down & pod deformity. Last year, however, production only slowed for 2 weeks, with very little deformity... and that was with the second-highest average temperature for the summer (shoots down the heat = deformity theory). So perhaps weather & location, as well as variety, influence the timing & degree of slowdown.

Maybe next year we could set up a nation-wide experiment? Perhaps with several widely-adapted varieties, such as "KY Wonder", "Fortex", and "Rattlesnake". Pruned vs unpruned, with evaluation for yield, quality, and tracking of bearing cycles for both methods.

>Perhaps biomass & hormones are just two ways of discussing the same process. Or, if not the same process, so closely intertwined as to make no never mind.

We say that the plant "knows" when it has reached it's peak biomass capacity. But just how does it know? There has to be a signal. And it's likely that the production of these hormones is what does it.

The open question is, just how many drying pods can the plant hold while still supporting growth of new, quality pods? I suspect this varies greatly between varieties.

All I grow are heirlooms, so can't speak for any of the more modern varieties. But among those I grow, I'd say the number is higher than we would initially think. Perhaps as much as half the biomass capacity.

This is just supposition on my part based on informal observation, as I have not studied the matter.

I wonder, too, if size of the vines contributes to the equation. I notice that my Striped Bunch---a true half-runner---are more likely to set flowers while seed is drying then are some of the more rampant varieties.

I kinda enjoy my bean tower.

Here is a link that might be useful: Garden Pics