Why are pots typically wider at the top than at the bottom?

Question

Recently I needed a pot with a shape that would make it as stable as possible so that it's unlikely to tip over and fall from the shelve. I was looking for a pot as close to a cylinder as possible.

I had to search many shops before I could find a suitable pot, almost all pots are wider at the top than at the bottom, they are usually of truncated cone or rounded at the bottom form.

I'd like to know why the cylinder shape is so rare. Does having a less stable form than most pots have give any advantages?

Answer 1

With ceramic pots, I imagine it's easier to get sloped sides cleanly out of the mold when manufacturing. This may be true of some plastic manufacturing, too.

When transplanting, it's easier to get the plant out, complete with all the soil, if the sides are sloped. For seedlings, the reduction in root hair damage may also be significant. Once it's free from the bottom, there's no contact with the sides, if you pull out straight. With a cylinder, the soil is scraping against the sides all the way up. If the sides are really slippery, like most plastics, this doesn't matter too much.

This makes the most difference when transplanting seedlings, and plants where the pot is still larger than the plant needs. For seedlings, the reduction in root hair damage may also be significant. At the other extreme, larger root-bound plants will usually hold the soil together regardless of pot shape.

As for styrofoam seedling trays which are often cylindrical - the shape is certainly a problem for me. The higher friction of them makes getting seedling out of them cleanly a lot harder than with other materials. The shape compounds the problem. Tried them, didn't like them. They're probably fine for growers for whom the seedlings are always root-bound before transplanting.

Answer 2

The simple answer is to minimize the size in transit. Conical shapes slide into each other producing a neat, stable stack. Cylindrical shapes can’t slide into each other, which effectively quadruples the size of the pot.

That and as Ed Staub said, it makes plant removal a bugger.

Answer 3

Don't know much about gardening but I happen to know something about ceramics.

If you look at the history of ceramics e.g. vases or urns from the classical world, you will see that virtually all traditional/pre-industrial are some variant of a round cone. This is because the firing of ceramics creates enormous compressive pressure on skin/surface of the object. It is this compressive pressure that makes the ceramic strong for its weight.

The compression tries to press the vessel into a sphere. If you try to create a cylinder with ceramics the firing compression will squeeze most strongly right in the middle causing it to break. That's why it's so hard to make ceramic pipe and most historical ceramic pipes where not true cylinders.

Creating a conical shape creates some usable cylindrical space while directing the compression force in a strong circular vector that works something like an arch.

Stack-ability and plant management are most likely simply nifty side effects of ceramic processes.

Answer 4

I don't know that it's the reason they do it (I like TechZen's answer), but if pots were the same width at the bottom as the top then the saucer to catch the water would be wider than the top, which might be an issue for some people in some situations.

Container shape does influence some things, however:

Plant roots typically grow around the sides of the container rather than in the middle of it (maybe they like the extra oxygen). So, with a narrowing bottom, the roots may be exposed to more water, since it will fall to them from multiple upward points, at multiple rates instead of one.

I've also noticed that a tall cylindrically shaped container (a big soda bottle) had some major issues when I tried a plant in it (without drainage). The water made the soil float and drown the plant, without wetting the soil effectively, whereas the same amount of water wouldn't do that in just any container without drainage. I can't say whether adding drainage would have solved that particular problem for that container.