Get Bottle Trees out of Containers and Into the Ground

Q.
I am growing healthy bottle tree in a very large concrete container in
Queensland, Australia. The lower branches have been turning brown and then
falling off. Is this typical behavior as this tree grows taller or a water
problem? I am watering maybe 2-3 times a week using a handheld hose.  We are currently moving into our summer with
very warm to hot days.

A. All bottle trees (Brachychiton populneus and B. rupestris) growing in containers or pots presents its own set of problems (primarily heat,
soil, and water management) that growing them in the ground does not. I would
encourage you to think about planting in the ground if possible. Our bottle trees get big! So do Australian-type bottle trees!

Bottle Trees in Australia are Different Than Ours

            In Australia they sometimes call Brachychiton
rupestris a bottle tree. In the US, Brachychiton populneus is called
the Australian bottle tree. Both are natives to you but rupestis gets the
typical bottle look when younger while populneus gets a bottle shape that is
not as exaggerated. Dropping of lower limbs may be natural with that species if
they get eight years old. Hard to know since it depends on how old it got and
if the trunk was starting to swell.

Be careful when putting bottle trees in lawns. These trees don’t like lawns because lawns are watered too often. They are native to Queensland and Victoria, New South Wales, Australia and prefer soils that have good drainage and are not watered frequently.

Heat Management of Containers

I would suggest growing
them in double pots. The outside container creates shade and acts like a heat
protector for the roots. When the sun shines on the pot directly it can damage
or kill about 50% of the roots due to heating up of the soil.

            The surface of dry containers, when
the air temperature is 105F, is about 175F. Plant roots can handle temperatures
at about 135F. I tell people to water their plants just before the heat of the
day. As trees get older their roots occupy more of the container and this can
be a significant problem during the summer. Root growth occupies space.

Bottle trees, Brachychiton populneus, can sunburn on upper branches because of its thin bark. It is easily vandalized as well.

Soil
Management of Containers

Soil in containers “wear out” (depletes) in about five years and
should be changed or amended every three to five years. I would discourage you
from growing a large tree (bottle trees mature at about 50 or 60 feet) in
containers. Most containers are not large enough to handle tall trees and their
roots as they get larger. Containers are better suited for smaller plants like
limes, calamondin, and finger limes.

Water
Management

The usual problem with bottle trees is poor drainage and watering
too often. The roots will rot if they get a continuous water supple and the
soil stays wet. Digging them up when roots are rotting is usually accompanied
with a putrid smell as the roots rot. When watering by hand about one fourth of
the applied water should come out the bottom to remove salt.

            If you can’t control the water in
the soil (drainage) or the application of water, then plant the tree on a
“hill” that is at least one foot higher than the surrounding soil and six feet
in diameter. Cover the soil with a 2-to-3-inch layer of surface mulch, either
rock or woodchips.