Hemlock woolly adelgids

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Ever see one of those Sci-fi movies where the off spring of genetically superior parents are so perfect they can ward off all diseases and have amazing intellect?  Sound crazy?  Well, yah a bit when it comes to humans, but in the plant world, hybrids are created all the time in attempts to not only increase the benefits of a plant, but its resistance to problems or pests.

One such example was noted in the December 2010 issue of the “Nursery Management & Production” magazine.  Yes, I finally got around to reading my December issue in April 2011.  This article, titled, “Hemlock hybrids resist destructive pest” caught my attention because I love offering hemlocks to clients as an option for hedges or privacy borders, but it has one drawback – the woolly adelgid insect (Adelges tsugae).  Tsuga canadensis, the Eastern or Canada hemlock is susceptible to it.  So is Tsuga caroliniana (Carolina hemlock).  Both hemlocks are native to the U.S. and applicable to our CT planting zones.

Hemlock woolly adelgid (HWA) is caused by female aphid-like insects that feed off the plant.  It is considered to be one of the major invasive pest threats in New England.  You can spot it on the hemlocks by a cottony material that stands out against the dark green needles.  This wool-looking sign consists of egg masses laid in early spring on the twigs.  If you don’t spot the white stuff right away, you later may notice your hemlocks turning grayish green first, than brown as they weaken from the feeding process.  Sometimes the HWA invasion can lead to the death of the tree.

However, all is not lost if you are forewarned of this potential threat.  A good professional designer or landscape installer with a conscience should advise their clients about this possibility if recommending this tree.  The maintenance required to control HWA involves spraying horticulture oil once in early spring before new growth begins, and again in the fall.  This will offer excellent control and is easy to do, but, with that said, you have to be willing to do this or at least be aware of the HWA problem so you don’t leave your hemlocks unsupervised.  If an infestation occurs and becomes really bad, the tree may need to be removed entirely.  I make sure to inform my clients of this if they aren’t already aware of HWA. 

In the plant world, bad scenarios like this don’t always happen though.  I have seen many plantings of hemlocks that have never experienced the woolly adelgid problem at all.  It is kind of curious to me, but then again not.  When hemlocks are grown in the right place and have a healthy start, chances is minimized for problems.  Plants in a weak state are larger targets for insects.  And with new hybrids on the horizon, homeowners will have new choices if they don’t want to risk HWA.  But we are going to have to wait.

After reading the aforementioned article, I contacted Sue Bentz, a horticulturist of the USDA’s Agricultural Research Service, to inquire a bit more about their process.  She indicated their hemlock hybrid trails are still in the testing phases.  The test hybrids are being cloned for out-planting trails to evaluate hardiness, landscape merit and whole plant susceptibility to HWA.  They hope to establish the first field trail this year.  The evaluation process may take several years.  Sue continued to explain, the hybrids between T. chinensis and T. caroliniana (and other Asian species and T. chinensis) are showing greater resistance to HWA than the susceptible species. 

Until these resistant hybrids are available on the market, please don’t be afraid to purchase T. canadensis.  It is a wonderful option for our CT planting zones in shady areas.  It is useful in the landscape as a larger hedge plant or one to block an unsightly view.  Its wispy, soft needles allow the branches to interconnect creating a nice green fence, plus the plant stays full to the ground level so there is no bare spot below.  It also grows on both alkaline and acidic soils.  And T. caroliniana (Carolina hemlock) is also beautiful, an upright tree with short and pendulous branches.  It looks very similar to T. canadensis but its needles go around the stem and it has larger cones.   Besides the HWA concern, the only other consideration is to not plant them in exposed locations. Both do not care for strong winds. 

To learn more about New England pest threats, check out this website:  http://www.ProNewEngland.org. Cathy T

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