This is a great question. It's interesting to
think about how various treatments could affect
plant growth, both positively and negatively. Once
you start thinking about exactly how to test and
measure the effect it can get pretty complicated!
The effect of caffeine on plants is not very
There is a lot of information (mostly
anecdotal, which means not a well controlled
experiment) about how coffee grounds can be used
as a soil additive or in composting in
general, but coffee grounds contain a lot more
than just caffeine, so it can be hard to
separate out the effects of just the
caffeine. I actually compost my coffee
grounds, but they get mixed in with a lot of other
The coffee plant itself is pretty
interesting - when it's leaves drop onto the soil,
they can affect the pH and even inhibit growth
of other plants around the coffee plant - but
is it the caffeine in the leaves, something
else, or a combination? You might be aware
that our local pepper trees have this same
capability by the way!
Luckily, with regard to your question, you can
use purified caffeine and apply it to plants to
monitor the effects, everything else being
equal. Scientists have done this experiment,
monitoring mainly germination (sprouting from the
seed) and growth (cell division) at the root tips.
They do a "dose response" test using
various concentrations of caffeine and also "mock"
(control) treatments for comparison. In almost
all cases, caffeine, at a high enough level, tends
to inhibit plant germination and growth. But,
exact results depend on the plant species, how and
when the caffeine is applied, and for how long.
It is amazing how a simple question can turn
into a really complicated investigation.
Keep asking great questions!
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