Trees have many types of thread-like growths on buds, leaves
and roots, but none of
them are hairs. Mammals have hair and fur. Trees
have “trichomes.”

Fuzzy trees

Take a close look at a sycamore leaf. It can be so fuzzy on
its underside that the
dense trichomes can be rubbed off into small balls of
fluff.

Sycamore leaf trichomes can cause allergic reactions and
respiration problems. The
fuzzy materials look like hairs, but they’re not.

All kinds of
trichomes

Trichomes can be all over the tree — from leaves to root
tips. They’re part of the
surface structure of tree parts. They are formed from the
outermost layers of leaves, buds
and roots.

Some remain alive for long periods. Others quickly die,
leaving an empty shell behind.
Trichomes are unique for most taxonomic groups of trees and
can be used for
identification.

Trichomes can be tall or short, thin or fat, big or tiny.
They develop from a single
cell or many cells on new tree surfaces like absorbing roots
and leaves. They can be
thickened at the base or have a large bulb at the end. They
can stick straight up above
the tree surface or recline on the surface.

Some trichomes are temporary, lasting just weeks, while
some are permanent fixtures on
tree surfaces. Trichomes can be disposable, breaking apart or
falling off over time.

On new roots, trichomes are found just behind the growing
tips. These root trichomes
are sometimes mistakenly called “root hairs.” Trichomes can
form round
containers that hold water on leaf surfaces.

Trichomes’
purpose

Some trichomes are glandular. These have various materials
that accumulate in or on
their tips. The stickiness of butternut leaves and fruits come
from glandular trichomes
exuding materials.

Glandular trichomes also serve important waste removal
functions in trees. Some species
of trees which grow on alkaline soils or near the ocean
transport salts and heavy metals
onto the trichomes’ ends. This secreted material prevents
tissue damage and helps ease the
washing away of excessive salts.

The underlying purposes for trichomes are as diverse as the
trichomes and tree species
themselves. Roles revolve around light absorption and
reflection, tissue protection, water
conservation and microbial interactions.

Young leaves of many species use trichomes to shade
photosynthesis cells until they are
fully functional. Some trichomes tangle, disrupt and confuse
bugs and prevent some types
of insect injury. Others, with defensive materials at their
ends, touch and stab at insect
visitors.

The tangle and mass of trichomes interfere with chewing-
caused injuries. Trichomes can
help slow water loss from leaves, too, by forming a thick
layer of higher humidity around
each leaf.

Trichomes on absorbing roots help take up water and
essential elements. Some root
trichomes act as avenues of colonization for beneficial fungi
and bacteria in the soil.
Unfortunately, some pathogens use trichomes for attaching to
the root.

Go feel some
trichomes

Feel some of our native trees. Many have trichomes of one
form or another. If a tree
has trichomes at all, it usually has them on the underside of
leaves.

Green ash, sycamore, Southern magnolia, red mulberry, red
elm, live oak, black oak,
chestnut oak, post oak and river birch are just a few of the
many trees that have
trichomes.

One common exotic tree, the royal paulownia, is so densely
covered with thick trichomes
that its leaves feel like thick felt. An old common name for
paulownia is
“cottonwood” because of the dense, cottony texture of the leaf
surface.

When you feel a leaf surface on a tree and the texture is
hairy, rough, bristly, or
silky, you’re touching trichomes. Try not to call them
hairs.