Austrees and Hybrid Poplars

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

They often sound too good to be true... advertisements for fantastic plants and super trees. Is it?

The Austree is actually a hybrid willow tree.  It's a cross between the Hankow willow (Salix matsudana) and white willow (Salix alba). Willow trees are fast growing trees and the Austree is no exception.  The advertisement indicates that they're "very fast growing" and can grow as much as fifteen feet a year.  I don't doubt this one bit, especially with our local summer sunshine and heat. The advantages to a fast growing tree are obvious, but fast growing trees tend to have soft wood that breaks easily and is prone to wood rot. 

Fast growing trees also tend to have large, invasive roots systems, and the Austree is again no exception.  The ultimate mature height of the tree is 50 to 70 feet and the literature suggests that the roots can extend into the soil two to three items the height of the tree.  Austrees are used to stabilize slopes and minimize soil erosion in gullies.  This implies a very invasive root system... it has to be to help hold onto the soil.

The literature also indicates that the Austree has problems with alkaline soils where the pH is above 6.5.  Most of our local landscape and garden soils are above 8.0 which means that the Austree may have problems here.  This can be ameliorated by fertilizing with an acidifying fertilizer, such as ammonium sulfate or sulfur coated urea, on a regular basis.  Failure to amend the soil  could mean poor growth and performance.

The Austree is also susceptible to the same pests and diseases as other willows growing in our area.  The two common pests that I've seen on willows are aphids and mites. These can be controlled with applications of pesticides, but it may be difficult spraying large trees.

What it all comes down to is that Austree should not be considered as a shade tree for a regular- sized or small home lot.  They should work well for stabilizing slopes, creating a dust and noise barrier, acting as a windbreak, acting as a wildfire break, or providing a privacy screen.  They should also be useful in creating shelterbelts and wildlife habitat. 

The important thing to remember is that they do have an extensive and invasive root system.  They should be located where this will not create a problem with driveways, sidewalks, septic systems, buildings or other structures. Another thing to know about making the Austree into a hedge or windbreak is that they should be planted about three feet apart in a single row planting and five feet apart in a double row planting.

The fact that these trees have lots of roots also implies that these trees like lots of water...  and that's true.  The Austree people recommend that after the seedlings are established they should have a good soaking every three days.  In fact, they recommend irrigating with drip irrigation to make this easier.

Planted in the right place for the right purpose, the Austree might just have a place in our local landscape.  However, I'd recommend staying away from it as a shade tree for the home landscape.

Hybrid poplars are another tree being touted by many because they grow so fast.  Just look at the hybrid poplar plantations that are being grown for pulp, fuelwood, lumber, and plywood.  It's truly amazing just how fast some of these hybrids grow!  Unfortunately, the hybrid poplars are not well suited as a shade tree for the home landscape.  The same reasons that make the Austree willow unacceptable also apply to the hybrid poplar... they have invasive roots and brittle wood.  Large limbs of brittle wood and potential wood rot lead to limb breakage and potentially hazardous situations.

However, the hybrid poplars do grow fast and are less susceptible to some diseases than Lombardy poplar, the variety traditionally used for windbreaks.  These new poplars are preferable over the Lombardy for windbreaks because of their rapid growth, their dense green foliage, and the absence of root sprouting.  The use of male clones can also avoid that nasty problem of blowing cotton.

The Rocky Mountain Austree Company offers two interesting hybrid poplars. One is called 'Rapid Merlot'  which leafs out in the spring with deep green and maroon leaves.  The leaves turn darker green during the summer and change to maroon again in the fall.  The Austree company notes that this is a good tree for windbreaks and can grow from 10 to 15 feet in one year.

`Gold Panner' is also a hybrid poplar offered by Austree Co.  This one only grows five to eight feet a year and is recommended for shade tree use instead of windbreaks.  Keep in mind that this is still a poplar tree and with typical poplar problems. Spring leaves are large and golden-yellow in color with purple stems and veins.  They tend to change to a lime green in mid-summer and then back to a brilliant gold in the fall.  I like my trees a dark green so I'd personally stay away from this tree, but the coloring does sound distinctively different. 

Until recent years local poplars and willows weren't bothered with many serious pests.  However, there is an increasing problem with borers on both poplars and willows in this area.  One borer is the "poplar and willow borer."  There is also the carpenter worm, which attacks both trees too.  These can structurally weaken limbs and can lead to the death of the trees.  Control involves keeping the trees in good health and applying a pesticide spray at the recommended times.

Another insect problem that's becoming widespread on poplars is the lettuce root aphid.  This aphid causes a reddish, flask shaped gall on the leaf stems.  It was first thought to be harmless, but apparently high populations of the pest are capable of causing damage and tree decline.  This insect can also be controlled with sprays... if one has the equipment to spray tall windbreak poplars.