Thursday, October 23, 2008

Tree-of-Heaven (Ailanthus) Control in Virginia

Non-native invasive plants threaten natural ecosystems because they can replace diverse native plant communities with monocultures. Tree-of-heaven (Ailanthus altissima) - the 46th most abundant tree species in the Commonwealth - is considered by many to be the most serious non-native woody invasive plant in Virginia.

A study was initiated in early 2006 to evaluate the effectiveness of herbicide treatment in combination with different harvesting strategies. This was followed by separate tests of the same application technique earlier (March) or later (September) in the 2007 growing season.

All herbicide treatments in the tests involved a tank mix of triclopyr ester (Garlon 4) in a hydrocarbon / limonene oil carrier (JLB Oil Plus) at a ratio of 1:3 triclopyr : oil. The mix was applied using a Solo backpack sprayer. Individual Ailanthus trees were sprayed around their entire circumference approximately 12-18 inches above ground. The objective was to determine whether treatment effectiveness varied with season of application.

Mid-Season Test
Three herbicide application strategies – basal stem spray followed by chainsaw harvest one week later, basal stem spray followed by chainsaw harvest four weeks later, and chainsaw harvest followed immediately by a cut-stump treatment - were compared to harvesting with no herbicide treatment. Each of these four treatments was applied to ten stems in each of three dbh size categories ranging from 1 to 16 inches.

The pre-harvest herbicide treatments were applied on June 5-6, 2006. These stems were then harvested either one week later (June 12-14, 2006) or four weeks later (July 5-6, 2006). Also on July 5-6, 2006, the stems receiving the cut-stump treatment and those left unsprayed were harvested.

The number of stump sprouts was tallied on September 25, 2006. All of the herbicide treatments had worked well to that point; fewer than ten percent of stumps from the treated stems had resprouted, compared to nearly 70 percent of those left unsprayed. Sprouting was evaluated again in July of 2007. The earlier results appeared to be holding, as only those stumps that did not receive the herbicide treatment had sprouted, and the largest trees had the most stump sprouts (see Figure below). Since there was no resprouting even in the plots where the trees were cut a week after treatment, it appears they were dead within one week.

These results show that a mixture of Garlon 4 herbicide in an oil-based carrier applied as a basal or cut-stump spray shortly after leaf development in the spring is a good treatment for removing tree of heaven up to 16 inches in diameter.

Early- and Late-Season Tests
Results of the early and late season application are shown below.

The early-season (March 28, 2007) trial was established in Nelson County, VA. Fifty-five Ailanthus ranging from 2 to 16 inches in dbh were treated. Within two months, roughly half of the trees had died while the other half had most or all of their foliage intact and healthy. Virtually all (96%) of them had died within 6 months.

The late-season (September 12, 2007) study plots were established near Batesville in Albemarle County, VA, where 65 Ailanthus ranging from 2 to 11 inches in dbh were treated. By the end of May 2008, only 40 percent of the trees were dead; the remaining 60 percent had stunted leaves on a few branches. By August, only 18 percent of the trees still had any leaves, and it appears likely (due to their poor condition, tiny crowns, and secondary infestations with wood boring insects) that all of them will die. At this site, we have observed a (not unexpected) relationship between tree size and the amount of time required for control. Only 30 percent of trees smaller than 6 inches in dbh had any leaves in May compared to over 70 percent of the larger trees. And by August, all of the trees less than 7 inches in diameter were dead while 18 percent of those larger than 7 inches still had small tufts of stunted leaves.

Tree-of-heaven was consistently controlled by basal sprays of triclopyr ester (Garlon 4) in a hydrocarbon / limonene oil carrier (JLB Oil Plus) at a ratio of 1:3 triclopyr : oil, but the response pattern varies depending on application timing. With a March application, about half of the trees leafed out although all were dead by the end of the growing season. After a June application, the trees were already at full leaf but wilted and were dead within one week after application. Following a September application, more than half of the trees leafed out in the spring although all of the foliage was severely stunted and very few branches had any leaves at all. By August over 80 percent are completely dead and indications are that none will be alive by the end of the season.

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