On 04/03/2012 12:59, AL_n wrote:
I have always ben sceptical of the theory that one gets the best growth
from potted plants by following the advice of using only a slightly larger
pot than before when potting-on. This doesn't emulate nature, which usually
seems to do things quite efficiently, when left alone, out in the wild-and-
woolly wilds...

If you follow the advice, it seems to me that you end up with an
unnaturally dense root structure, with an unnaturally small amount of soil
between each root from which to extract nourishment.

So is the "next-size-up" advice good advice, and if so, why?

Thanks,


Maybe I'm playing Devil's Advocate (or just a fool...), but having read
the various replies to your question I think that repotting in a
slightly larger container is one of those old chestnuts which everyone
follows without really knowing why. Actually, it may not be that old.
Looking through some old gardening books (50 - 100 years old) I see no
mention of restricting pot size when repotting. Maybe it's because
there weren't so many sizes of terracotta pot in those days, compared
with the plastic pots we have now. One book does mention putting small
seedlings into 3" pots, then progressively into 5, 7, and 9" pots (was
that "one size up" in those days?). Another mentions putting small
primula seedlings into 5" pots "as that is the best size to show them".
That's quite a jump; it would take some time for the seedling's roots to
wander through the pot.

So what good reasons are there to not overpot? One is to save compost
over the short term. Another, lack of space - I can't see much point in
putting a 10 cm seedling in a 25 cm pot. The greenhouse would soon fill
up! As to the compost going sour, stale, or whatever, I'm not
convinced. Maybe the best thing to do would be to put a few earthworms
in the pot - they'd turn the soil over and aerate it. As to the compost
being too wet in an "overpotted" plant, well, some plants might suffer a
bit and lose some roots to rot. But a small pot, full of roots and with
very little remaining soil, is far more likely to lead to the death of a
plant when it quickly dries out (eg in hot, windy weather). IME I have
lost far more potted plants to drought than drowning. In a bigger pot
with more compost they might have survived.

If you search for "Repotting" on the RHS website you'll turn up over 150
hits. Checking a few reveals the "one size up" recommendation without
giving any reason. Intriguingly, the one referring to "Growing fruit in
containers"
(http://www.rhs.org.uk/Gardening/Grow...uit-containers)
makes no such recommendation. Why? Are these plants such gross feeders
they'll push their roots through a much bigger pot at an express rate?

Maybe it's an experiment I'll carry out one day - potting seedlings into
small, medium, and large pots to see how they get on. Perhaps someone's
already done it. If so, I wonder what they found.

--

Jeff