How Trees Know When to Turn: The Science of Fall Color 🍂🍁🍃

The leaves don’t wait for frost—they’re already counting the night. We dive into the quiet timing system inside trees that decides when the colors ignite, why some years burst into crimson while others fizzle to brown, and how weather and latitude shape the show right above our heads. It’s a guided tour of photoperiodism, phytochromes, and the pigments that paint fall—carotenoids hiding under summer’s green and anthocyanins that arrive late to set maples and oaks on fire.

We break down how plants measure darkness rather than just daylight, why the red vs. far‑red balance matters, and what “cool nights, sunny days” really does to sugars in leaves. Then we follow the story to its elegant conclusion: the abscission zone, a microscopic tear line that lets a tree reclaim nutrients and drop each leaf cleanly. Along the way, we connect the science to practical gardening—how healthy soil, reduced chemical use, and smart fall cleanup can protect pollinators, save water, and set up stronger color displays next year.

This is a nerdy, satisfying look at the biology behind peak color season, told through the lens of our Minnesota landscapes and the broader mission to build resilient, pesticide-free yards. If you’ve ever wondered how trees “know” when to turn, or why reds pop only in certain years, you’ll come away with answers and a toolkit of simple, high-impact steps you can put to work at home. Subscribe, share with a friend who chases leaf-peeping weekends, and leave a review to help more neighbors find the show—and consider joining our community to grow healthier landscapes together.

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Episode Transcript

SPEAKER_00 (00:06):
Hello and welcome my friend to the Minnesota
Gardening Podcast.
My name is Brad Tadkey, and I amthe host of the podcast, and I
am honored to have you here withus today, where we are working
together as Minnesota gardenersto grow a community of Minnesota
homeowners switching to healthylandscape practices so that we

(00:27):
can stop the exponential rise inpesticide-induced cancers,
reverse the collapse ofpollinator and songbird
populations, and save our cleanwater.
We are working really hardtogether to get that done.
And it's no small task,obviously, and we are in the
middle of our October membershipdrive.

(00:49):
And today is the final day ofour first time of offering
quarterly memberships.
And so we finished up our drivelast week with annual
memberships for our cheapestprice that we have all year
long.
And uh today ends theweekend-long push for quarterly
memberships.
And a quarterly membership isevery three months, and uh it is

(01:11):
$77 to join Minnesota Gardening.
And we work at MinnesotaGardening to help to get to
healthy landscapes.
And so if you are usingchemicals, if you're using
water, if you're not feedingpollinators and songbirds, and
using your home landscape as amechanism to make the world a
better place, we're here for youif you want to make those

(01:34):
changes and start to change froma uh uh toxic landscape to a
detoxing landscape to anourishing landscape to a
healthy, thriving landscapewhere we have eliminated
chemicals, feed pollinators andsongbirds, saving water and
storing carbon at our homes.
So that's what we do.
Just little things here atMinnesota Gardening.

(01:56):
And this is a very personal uhcrusade for me.
I really want to make sure thatwe are doing the right things
and our home landscapes for ourkids, for our grandkids, for the
future of our environment andeveryone else involved.
So I hope you join MinnesotaGardening.
Uh, today is the final day ofjoining at our quarterly price

(02:16):
of$77 per quarter.
So I hope you join there.
Today we are in a solo podcastepisode and we are taking a deep
dive into fall colors.
And so this is kind of a nerdy,wonky episode for you.
Um, oh, I just realized if youwant to join, head to
MinnesotaGardening.com.

(02:37):
That is where I you're justlistening.
You're not uh able to see what Isee here.
And uh, if you would like tojoin as a Minnesota gardening
member, head toMinnesotaGardening.com.
All the information is rightthere, uh, and you can head in
that way.
So MinnesotaGardening.com.
Now, today we are talking aboutfall colors.
And so I get often questionsabout how this happens, like

(03:01):
what the hell, right?
Like, how how do trees andplants know that it is fall and
they don't just wait for it tofreeze and then fall off?
They actually have a processthat we all see and we all know
um that is a changing of theseasons in our landscape plants,
and that's the fall colorchange.
And so that is the brilliant,you know, yellows and reds and

(03:24):
purples that we see.
How do those plants know beforeit gets cold how to change and
when to change?
And so it's a a process thatwe're gonna walk through today
called photoperiodism, and so uhit's the plant's internal clock
that it knows how uh long thedays and how long the nights

(03:47):
are, and once it hits a triggerpoint, and all plants are
slightly different for thattrigger point, um, then they
start the process of shuttingdown for the winter.
And so to me, this is along withlike we've talked this month all
about soils as uh MinnesotaGarden members, and it is just
fascinating to me how plantsknow and do these things.

(04:10):
Do they think?
Do they have a brain?
Do they know?
Uh do they have memory?
What does do all those thingseven mean?
And uh, so it is really, reallyfascinating to me that plants
are able to do these kinds ofthings.
And we don't really know, likewe know the mechanisms, but we
don't know exactly how they putit all together and get to uh

(04:31):
the end product that we see asthe just uh glowing sugar maples
that we see in our parks and inour landscapes that are just
glowing yellow, the uhbrilliant, bright red uh oak
trees.
We don't know why they turnbrilliant bright red and what
the what the point of that is,and we'll get to that in just a

(04:51):
little bit.
Um, but we'll start with theplant's internal clock, and it's
uh a thing calledphotoperiodism.
And uh photoperiodism happens uhbecause plants are able to
recognize and sense light, andso we intuitively know this
because plants will uh you knowgrow toward the light, they will

(05:14):
grow to maximize their leafspace that is exposed to the
light, so for photosynthesis.
Um but we don't actually uhoften think about plants as
seeing, and I'm using air quoteshere, seeing the light, but they
absolutely can.
They have photoreceptors umcalled phytochromes that are in

(05:35):
their stems, and thosephytochromes are able to sense
light.
And so they're able to senselight um in the shutting down
period in the fall, they're ableto sense light as to timing as
to when to start up in thespring.
So when we get a uh a warmperiod that the plant doesn't
just start growing because it'swarm out, the plant starts

(05:56):
growing because they're actuallyseeing how much light there is.
And recently there was aninteresting um, at least what I
find is interesting, a reallyinteresting uh research uh done
about what actually the plantssee.
And what they see is notnecessarily the daylight, but
they see the lack of daylight.

(06:16):
And so they have two types ofreceptors in their phytochromes.
And those uh phytochromes, theymeasure the length of the
nighttime.
So what uh in the fall, whenleaves are starting to shut
down, what they've actually seenis that the nighttime gets
longer, and that changes withlocation, right?

(06:38):
So it changes at a differenttime, whether you're in uh
Canada, whether you're inMinnesota, whether you're in
Iowa, whether you're in Florida,those are all differences in uh
nighttime length, and so thetrees see that happening, and
then they start to uh changebased on that.
And so there are two types of uhpigments that those plants uh

(07:03):
have, and those are uh um phytosorry, I was got lost in my my
head there.
Um, those are phytochromes thatuh see far red light and see red
light.
And so the red light is activeduring the daytime, and uh the
far red light is active duringthe nighttime, and so they have

(07:24):
a chemical that breaks downthrough that time, and the
amount that chemical breaks downthrough the nighttime periods is
what the plant measures andknows when it has turned into
autumn, when uh that uh far redlevel has dropped below a
critical threshold, it triggersthe start of dormancy and leaf
change, and so that it differsfrom everywhere in the country

(07:48):
based on your latitude and yourlocation.
And so it's just really, reallyinteresting to me how this all
happens.
And so that's what triggers thechange for the plants to realize
that fall is happening and thatthey need to start that uh
dormancy process for the season.

(08:08):
And what happens next is evenmore interesting, I think.
So there are a couple differentways that uh we'll focus mostly
on trees here, that trees handlethis, and so what happens is the
uh uh daytime length is whattriggers the process starting,
but then as we see some years wesee bright uh brilliant, bright

(08:31):
uh colors, and other years wesee pretty dull brown colors and
those kinds of things.
Uh, the weather and how fast theum the photosynthetic materials
break down in those plants iswhat causes the different um uh
amounts of light and so ofcolors that we see each year.

(08:54):
And so if it is uh cool nightsand uh warm sunny days, we get
the brightest possible colors inour um fall colors.
If it is really, really hot,like we had this year in 2025,
uh, those fall colors will be uhdrowned out a little bit because
they are not getting what theyneed in order to break down uh

(09:17):
well.
The process happened way toofast.
And so inside the leaves isreally, really fascinating.
So uh we all learned, probablyI'm guessing, in uh middle
school and high school inscience that uh the leaves are
generally, and there's twodifferent types here, uh the

(09:39):
actual color of the leaves arewhat you see in the fall colors,
and those fall colors are justmasked by chlorophyll.
And so chlorophyll is theprocess for photosynthesis, um,
the location wherephotosynthesis happens, and in
those leaves.
And so what happens is the plantcreates chlorophyll, that

(10:00):
chlorophyll masks the color ofthe leaf, and so that is the
carotenoids, and so um thecarotenoids are yellows and
oranges, and those are what wesee um in the fall, and but it's
there all summer long, but wejust don't see it because it's
uh covered up by thechlorophyll, and um what they've

(10:21):
learned recently as well is thatwhen we have oak trees and uh
maples that are deep reds andpurples, those are the
anthocyanins that are showing upat that point, but anthocyanins
aren't present all year long.
Those anthocyanins only come inonce this trigger process
starts.

(10:41):
So we've got like uh a maplethat is uh we'll say a yellow uh
that has carotenoids in there,but then once the uh once it
wants to turn red, it buildsthese, it grows these
anthocyanins, which show up asreds and purples.
And those are more, much moreimpactful when we have cool

(11:01):
nights and a lot of sugars thatare trapped in those leaves.
And no one knows exactly whythis happens and why they do
them, um, but that is there, andso we've got those reds and
purples, which are newly createdcolors in that leaf just for
fall, and um again, fascinatingprocess.
Why trees do this exactly, wedon't really know.

(11:24):
Um, but it is there, and then uhthe more interest I keep saying
interesting.
I should probably stop usingthat word, but I uh I do really
think it's cool that uh plantsalso have what's called an
abscission zone.
And so that absission zone isthat point where the leaf meets

(11:46):
the stem tissue, so the thebranch, the twig, whatever it is
that that leaf is connected to,they uh they have to have a spot
where the leaf separates fromthat twig.
And so there's a uh a very clearlayer called the abscession
zone, where those cells, theyjust simply because of the fall

(12:07):
process and uh what wastriggered by this nighttime
length of uh and day length andthose photoreceptors, the have a
layer of plants that of cellsthat just dissolve and uh they
just uh self-destruct and theygo away.
And uh that is how a leaf fallsoff, is that those cells in the

(12:27):
abscession zone just uh poof aredone, and uh that's how the leaf
falls off and it's ready for thenext season to uh start growing
again at some time.
And when those phytochromes seethat the day length is uh that
the night length is shorteningagain, that's when the process
starts back up again forstarting to grow leaves and to

(12:50):
do all that work uh in reverseof shutting down for the fall.
So I'd love to know uh if youlike this nerdy, uh wonky side
of things.
We're gonna do this on thepodcast a couple different times
here, uh, but I just really amuh happy to have you here with
us today.
Go back through a little bithere, and so photoperiodism is

(13:13):
the kicker for what causes fallchanges to start happening in
our plant friends.
And so they have phytochromesthat are measuring the far red
light spectrum, and that uh theyare measuring how long the night
is.
And for that plant, once it hitsuh a length of night that is
growing uh to a certain amount,and every plant is a little bit

(13:35):
different, triggers the fallprocess to happen, and those uh
chlorophylls break down and thenthey move on to their fall color
that is uh uh impacted greatlyby weather and how much rain we
have had.
And then those leaves uh have alittle chunk that uh dissolves
and they fall off, and then weenter the winter season.
And so, with that, I justappreciate you being here.

(13:56):
I appreciate you listening andlearning about healthy
landscapes, and also encourageyou on our last day of the uh
quarterly um Minnesota Gardeningmembership uh push for October.
I just really hope you join.
It's 77 bucks a quarter and it'suh$77 per season and would love
to have you as a member ofMinnesota Gardening so we can

(14:19):
save thousands and thousands ofgallons of fresh, clean water.
We can reduce the use ofthousands of gallons of
herbicide in our landscapes, wecan feed literally millions of
uh pollinators and songbirdswith our landscapes, and we have
to do that together.
And with a hundred new membershere in the month of October, we

(14:40):
will do that.
And so please head to MinnesotaGardeningGut.com and join as a
member.
And I will see you in there verysoon.
Thanks a lot and have a greatday.