# What is the theoretical maximum height of a herbaceous plant stem?

Foreword: I'm posting this here instead of, say, Worldbuilding, because while it is based in a speculative concept, this is a question purely of biology as we know it instead of speculative biology under non-real conditions. I would address that later and elsewhere. And sorry if this is actually more physics than biology, I have trouble pinning these things down.

I'm trying to determine what the maximum height of a non-woody plant stem can be (i.e. I don't want the tallest flowers). I think the primary issue at hand is load-bearing and resistance to bending. So I found this 1997 paper by Schulgasser and Witzum that sort of discusses what I'm looking for. It suggests that the plant's ability to resist shear forces is a matter of how bundles of fiber are arranged inside the stem and of the plant's internal pressure, which affects how much weight it can stably bear. But I'm no biologist, so the paper is difficult to penetrate. I'm sure there are other issues limiting the height of non-woody plants, like the maximum height above the ground that capillary pressure can raise water. Considering all factors, what is the theoretical maximum height of a herbaceous plant?

I've found this question, which concerns the tallest real herbaceous plant, while my question concerns the theoretical. But I would like to extend that question - are there any examples of fossil herbaceous plants that are larger than the modern banana plant, and do they run up against the theoretical maximum?

• Unfortunately I think, despite your forward, unless you are interested in existing plants this feels a bit to speculative. Maybe you could reformulate and break up your question, e.g., ask what maximum observed root pressures are. Dec 29, 2016 at 21:05
• @BryanKrause Well, there is a difference between speculation and calculation based on established knowledge. The 1997 paper has several useful equations - I simply don't know how to use them. Dec 29, 2016 at 21:08
• I don't disagree about the relevance of calculations; I think the issue is that your question is putting together a lot of separate calculations and putting them all into a "maximum." For capillary transport of liquids, for example, a factor that is really important is the diameter of the tube. Well, one might be able to make a very small tube and go a long distance, but this introduces other issues, maybe not enough nutrient delivery to support a large enough stem for that height. Can you try to break your question down into the problems you are having with the equations in the paper? Dec 29, 2016 at 21:12
• I believe I see your point - that all the potential limits one could calculate based on various factors would be difficult to unify. Well, the larger issue is that I don't know enough about plant physiology to know what things I don't know. The paper is only one side of things. I'll try posing this question on Worldbuilding if it proves unworkable here, but I felt the chance of encountering a plant physiologist who could answer the question was a lot higher on Biology. Dec 29, 2016 at 21:18
• I'll let others decide definitively whether your question is indeed off-topic, but I think you could also reformulate a question to try to capture the things you don't know, without requiring speculation. Perhaps a question about "besides capillary limits and shear forces on stem fibers, what key factors limit plant height?" Not being a botanist myself, maybe that question is still too broad, but I think it's more in the right direction for this site. Hope that's helpful. Dec 29, 2016 at 21:39

Update: I have edited the answer for a better structure and for providing reliable references.

Some of the factors that influence growth, while keeping in mind that every species has particular needs:

light, temperature
• insufficient light leads to etiolation
water
amount of water and dry substances, page 185
water influences diameter
soil
wind
macronutrients
citokinin
genotype
heterosis
inbreeding
pinching the stem stops its growth and promotes lateral stems (I understand that you don't want to, but I list it here so that you can watch out accidents - involuntary pinching has the same effect)
root type
shelter near a fence or a wall

You might find some info as to what to consider in your calculations in this paper, although it's about trees.

Just for the purpose of knowledge, herbaceous vines like Ipomoea are also stems and they can grow longer than vertical ones. There may be more to add, but I'm only a Horticulture student, not a plant physiologist.

I strongly recommend to ask plant related questions on Gardening and Landscaping SE, a small community, but a very enthusiastic one. Also, hardly can someone become a good gardener without knowing at least the basics of plant biology and psysiology, so you have a major chance there of receiving answers in a few hours, not days.

Good luck with your project and I hope you'll return to tell us your conclusions. We will be happy to see you interdisciplinary approach.