I'd wondered for a long time how trees

sensed it was spring and knew to bloom.

We went to the National Arboretum here in D.C.

to find out.

You could sort of think of spring as

adolescence for a tree.

For us, that's not always fun,

but it's sort of the same thing for a tree.

It's a huge change.

When you look at a dormant tree

and what that looks like,

and what it looks like when it's in flower, and everything,

those are way different-looking things.

If you were from another planet,

you might look at that, and say,

that's not the same organism.

I'm Scott Aker.

I'm the head of horticulture and education

here at the U.S. National Arboretum.

We have a scientific mission primarily here,

but we are also a public garden.

How do trees know to bloom in the spring?

How do they know it's time?

Actually, it has more to do with the length

of the nighttime than it does with temperature.

Most people think that it gets warmer,

and we all learned this in grade school,

it gets warmer, and the buds start coming on the trees,

and then they bloom.

It's actually not as simple as that.

Plants have a funky little molecule in them

called phytochrome,

and it's like an hourglass.

The hourglass, or the molecules, is flipped

very soon after the sun goes down,

and then you think of time that it takes

the grains of sand to go through,

if there's enough time that

the whole hourglass empties,

the plant gets the idea that we should hold off.

The nights are still pretty long.

But then you have that one magical night that

not all the phytochrome is changed.

The bud gets a signal that it's time to get going,

and then we have a period of warmer temperatures

that help that development happen.

If you suddenly get cold,

those buds will get the signal to hang way back

if we have one night of drastic cold.

What're signs that people can look for to say:

Oh, this tree is about to bloom,

or this tree is blooming.

Here's what's going to come next.

Of course, the obvious thing is bud swell.

From a distance, you can see if something looks

thick and they're darker,

if you look closely, you'll see the little stamens

with the pollen on them.

Most plants have two different kinds of buds,

and you can see that very clearly this time of year.

You have big, fat ones that are flower buds.

They exist to reproduce.

And then, very often you have closer to the tip,

skinny, kind of slender ones that are vegetative buds

that will produce the growth.

The purpose of a tree blooming is one thing,

and one thing only,

and that is reproduction.

You can see it sometimes if a tree,

if we injured the lot of the root system on it,

or something,

it will bloom extra heavily,

and people will misinterpret that as health.

Actually, that's a: I know I'm going to die!

But before I die, I want to have lots of kids,

and perpetuate my line.

Trees are beautifully adapted.

I mean, even if you look at them structurally,

how they're put together,

they're more exquisitely, and strongly,

and efficiently built than any of our buildings.

If you're a tree, you can't move some place better

if conditions get bad,

so you have to adapt.

I guess I'm always really surprised at how

adaptable trees are.