I agree, that’s a great question. You’re doing a
great job if students are asking questions like
that! I’m not a plant physiologist, but here’s my
guess about what’s going on. It sounds like you
already explained that when a plant isn’t
getting light, it stops making chlorophyll.
Chlorophyll breaks down, so the yellow
(xanthophyll) and orange (carotene) pigments are
The pigment breakdown happens even faster in
sunlight (just like cloth will fade in the sun).
That explains why we have to move the lawn
Sometimes the grass gets all brown and dry in
the fall, but it depends on your climate and
the type of grass. This is senescence,
where the top of the plant seems to die, but it’s
really more like the leaves falling off a tree.
The underground part of the grass plant isn’t
dead at all. So what triggers senescence?
Daylength and temperature, plus the genetics of
The plant stops growing well before the first
freeze. A “killing freeze” or “hard freeze” (25
Fahrenheit or -4 Centigrade for at least 4 hours)
will not really kill the plant, but will destroy
cells above the ground. Ice crystals poke through
the cell walls, killing the cells, which will then
dry out and turn brown. A frost (with the
temperatures just below freezing) doesn’t damage
the cells unless someone walks on the frosty
My guess is that it’s all about timing.
Say the fall has been reasonably sunny and not too
cold. There may have been a frost, but not a
killing freeze, so senescence hasn’t happened.
Then the snow falls, covering the green grass.
Now the plant isn’t making more chlorophyll, but
the light is also not breaking it down. Freezing
may even help to preserve the chlorophyll that’s
there, just like spinach in the freezer (total
guess). Snow is a good insulator, so maybe the
snow-covered grass didn’t even get cold enough to
break down the cells and the grass will not even
turn yellow after it has thawed out.
I hope this is helpful. It might give you some
experiments to try with your kids. Let me know
what you find out!
Thanks for asking,