Bottle Tree and Leaf Drop

Q. Our bottle tree in Scottsdale is 11 years old and this summer had a crazy amount of leaves dropping. So, we watered longer but only once a week. It also gets some water when the lawn is watered, and right now, that is 3 times a week. The leaves are very, very light green, turning brown on the edges first and then completely brown and then dropping leaving it very sparse. Are we not watering long enough, or is it getting too much water?

Bottle trees sold in the nursery are not really a bottle tree. Yes it has a tapering trunk but the REAL bottle tree has a trunk shaped like a bottle! It may drop its leaves naturally when the soil is dry or just before it starts to flower in the late summer or fall.

The true bottle tree (B. ruprestis) has a much fatter trunk, shaped like a bottle.

A. These trees normally drop leaves just before flowering at the beginning of their wet season. In Australia where they are native, that would be just before our fall months. Many “south of the equator” trees don’t pay any attention to their new environment and flower or drop their leaves just like they were growing south of the equator. If leaf drop occurred starting around September and got worse a few weeks later, their leaf drop may be a response to longer nights, something totally out of your control.

Maximum/minimum temps in Scottsdale, AZ, USA.

Maybe it Was to Hot

Our normal summer temperatures are are about 10F hotter than their maximum high temperatures where they grow naturally. In their natural environment, the beginning of Australia's monsoon season mean cooler temperatures. With these high temperatures and low humidity and if there was a some wind then it would mean the plants and soils would dry out sooner. Your tree may have got hit with a double whammy; high temperatures and intense sunlight.

You did the right thing. You increased the amount of water they received by increasing the amount of water and not the frequency of water application. The only thing I would add is to increase the area where water is applied and not just the amount while all the time not letting the soil dry out between irrigations. My guess is the tree was getting most of its water (and fertilizer) from the lawn. That’s also probably were most of its roots are growing.

These trees like to have their "toes" in water but not submerged. Next time this happens, try watering your lawn a little more often. I think the amount of fertilizer the lawn is getting is plenty for this tree.

Bottle Tree with Droopy Limbs and What To Do

Q. I have several bottle trees that have grown well over the last years and one that has always had a slight “droop” to the branches. I’m not sure what else to do as all the other trees are doing great!

When young, many trees have upright growth. It is very common in young trees. Remember, Kurrajong (aka Bottle tree in the US) can reach 55 feet tall!

A. If the tree otherwise looks healthy there may be nothing you can do about it. Growers of bottle trees usually start them from seed. Propagating plants directly from seed creates a lot of tree variability in their growth habit, flower color, seed pod as well as the seed itself. The result is that some trees are more upright than others, some have different leaf color, and some more resistant or susceptible to disease or insects. Its genetic and nothing much you can do about it.

Trees grown from seed can show quite a bit of genetic variability in their shape, leaf color, overall size, size of acorns, leaf size and other visible traits. That's not even counting what you cant see!

Other trees propagated from seed include southern live oak, and most of our pine trees. So, you see, this variability can be good, or it can be bad if you don’t like it’s looks or it’s more susceptible to a disease present in your locale.

If you are not careful in your watering, the canopy can drop leaves or maybe some limbs might sunburn and die.

When trees are young, they oftentimes grow more vertically. They want to get taller and get taller than any competitors nearby. As they get older, vertical limbs become more horizontal. It’s possible this is what you see. When it has a full canopy of leaves then horizontal limbs are not a problem. The dense canopy of leaves shade these thin-barked horizontal branches. But if there is leaf drop from disease, insects, drought or normal leaf drop then watch for sunburn on these very susceptible limbs.

Because they are native trees, they don’t need a lot of fertilizer. Native plants are like that. They are not commercial hybrids that rely on more soil nutrients applied by fertilizer. This means applying a light application of high nitrogen fertilizer like 16-16-16 or 20-20-20 once a year in the spring just after new growth starts is enough.

Bottle Tree? Kurrajong? Which is it?

            I thought I would talk about “Bottle trees”. The reason for that? I get so many darned questions about them from readers. I get more questions from confused readers about this tree than any other landscape tree. These aren’t usually good questions; they are problem questions. All these problem questions makes you wonder if they should be planted in our warm desert at all! They should but choose a good planting location and water them carefully and not haphazardly.

The true Bottle Tree, Brachychiton ruprestis, has a bottle-shaped trunk. The Kurrajong, B. populenum, has a straigter trunk and is oftentimes called "Bottle tree" in the US. (Picture sent by reader in Australia where they are native)

What you are buying from the nursery (Brachychiton populneum) is not really a “Bottle tree” at all. The correct name is “Kurrajong”. The true bottle tree (B. ruprestis) is very different from the nursery version because its trunk is truly shaped like a bottle! It’s a beautiful oddity in the landscape. But the true Kurrajong hybridizes very easily with other types of so-called “Bottle trees”. The growers propagate “Bottle trees” from seed. Seed results from a cross between “mama” and “papa”. Unless the growers are knowledgeable, who knows what you are getting?

The Kurrajong, (B. populneum) which we here in the US oftentimes mistakenly call a Bottle tree, does not nearly have the trunk taper of  B, ruprestis. Picture sent by reader in US.

 The number one questions I get is about watering. They grow primarily in the dry interior of Australia. They are not a “swamp tree” and they are not a cactus but something in between. When you water them, think of them more like watering a palm tree (e.g., date palm) or even asparagus; they like to have their “toes” constantly in water but without the soil around them continuously wet or they will die.

Be very careful of overwatering Bottle trees, any of them. It is a delicate balancing act of giving them enough water vs watering too often when planted near or in a lawn.,

Leaf drop can be normal. It frequently will drop leaves just before  or if it’s not getting enough water (that does not mean water it more often!)

The horizontal branching and thin bark of the so-called Bottle tree can lead to sunburn if the canopy begins to thin due to leaf drop.

Plant in the spring, not during the hot summer months. Unlike palm trees, they don’t grow well after planting during the heat. Yes, they should be watered more often after first planting; maybe twice a week in the spring or fall rather than once a week. But after one growing season, apply a lot of water to a large area under its canopy but water less often. This means either apply the water slowly as in drip irrigation or build a mound or moat around the tree to hold the water so that it sinks in the ground. Applying a lot of water at once uses the same amount because “little sips are combined into one big gulp”.

Leaf scorch followed by leaf drop can occur because of watering too often or not enough water. Seldom is it from giving the tree too much water at one time unless the soil has poor drainage. Poor shade from the canopy can lead to sunburn of thin-barked limbs, particularly horizontal limbs.

The second problem I hear about from readers is sunburn or sun scorch on their primarily horizontal limbs. Sunburn on the limbs is for two reasons; the first is because this tree has very thin bark easily sunburned by intense sunlight and the second reason is due to where it’s planted. Never plant this tree in the hottest locations. It’s not a mesquite, Cordia or sweet Acacia.