56 DiTomaso and Kyser: Control of Ailanthus altissima ailanthone, which is most concentrated in the root and stem bark (Heisey 1996; De Feo et al. 2003). Control of tree-of-heaven can be difficult. Manual cutting or girdling the cambial tissue of the stem generally stimulates stump and root sprouting as a result of the loss of apical dominance, resulting in an increase in overall stand density (Kelly 2001; Burch and Zedaker 2003). Herbicides have also been tested for tree-of-heaven control in several studies. Hexazinone applied as a foliar treatment gave varied levels of control (55% to 93% at 2.4 to 9.6 kg·ha [2.1–8.6 lb·ac]) 21 months after treatment (Pritchard 1981). However, it is not registered for use near water and is not an option in riparian areas. Foliar application of the growth regulator herbicide triclopyr (Butler and Britting 1998) and glyphosate applied with a rope wick applicator to the foliage also provide control (Butler and Britting 1998). For more mature woody plants, low-volume basal bark treatments of triclopyr, picloram, and imazapyr were used to control tree-of-heaven in Virginia (Burch and Zedaker 2003). Two years after treatment, optimal control was achieved with a combination of triclopyr and picloram. Triclopyr and imazapyr alone or in combination also gave some control, but not as effectively as combinations with picloram. Picloram is not registered for use in California nor is it registered near water in any region of the country. Thus, like hexazinone, it is not a control option in most areas of the west where tree- of-heaven has invaded riparian sites. In a California study, Kelly (2001) reported a triclopyr basal bark treatment to be very effective (98% control) on tree-of-heaven. Large woody plants can also be controlled using a cut stump technique, in which the herbicide is applied directly to the cambial regions of the newly cut stem. It is widely rec- ommended that application of the herbicide be made within 30 min after cutting (Tjosvold and McHenry 1986). A com- bination of picloram and 2,4-D as a cut stump treatment was found to be ineffective for control of tree-of-heaven (Pritch- ard 1981). In contrast, triclopyr and glyphosate showed good results in one study (Butler and Britting 1998) with only a few plants resprouting after a cut stump treatment. However, in another report (Kitz 1997), these same herbicides did not provide effective control with a cut stump treatment. In our study, we examine several stem treatment tech- niques for the control of tree-of-heaven. We compare the effectiveness of three herbicides (triclopyr, glyphosate, and imazapyr), all with formulations registered for use in riparian habitats or systems adjacent to water. We evaluated the ef- fectiveness of cut stump, basal bark, stem injections, and stump injection treatments as well as untreated mechanical cutting. In addition, we examine the timing of removing trees after a stem injection treatment and the timing of herbicide application after cutting the stems. The objective is to provide land managers with the most effective option for tree-of- ©2007 International Society of Arboriculture heaven control in sensitive habitats near water and to expand the control options available in other infested sites such as urban settings. MATERIALS AND METHODS Treatment Sites The initial stem injection trial was conducted on the campus of the University of California, Davis, along the relict (now dry) south fork of lower Putah Creek at ≈15m(≈49.5 ft) elevation. This area has a warm Mediterranean climate with a mean yearly precipitation of 480 mm (19.2 in), most of it during the winter season. The subsequent study, designed to compare different her- bicides and application techniques, was conducted on Cali- fornia Department of Fish and Game property along Putah Creek in western Yolo County at ≈60m(≈198 ft) elevation. This region has a warm Mediterranean climate with a mean annual precipitation of 584 mm (23.36 in) (86% between November and March). Trees were located along a small fluvial terrace, 10 to 80m(33 to 264 ft) distant from the creek and 5 to 10 m (16.5 to 33 ft) above the mean creek level. This infestation has been present for approximately 20 years and originated with ornamental plantings at a home site a short distance upstream. The native vegetation consisted of blue oak woodland dominated by blue oak (Quercus douglasii) and foothill pine (Pinus sabiniana) in the uplands and by Goodding’s black willow (Salix gooddingii), California black walnut (Juglans californica), and seepwillow (Baccharis sal- icifolia) closer to the creek. Treatment Parameters The initial trial was conducted at two locations in the Uni- versity of California, Davis, Riparian Preserve. For all treat- ments at this site, individual plants were treated as replicates with eight replicates per treatment at each site. Tree-of- heaven plants were treated with one of two commercial for- mulations of imazapyr by a stem injection technique (Table 1). Because it has been reported that the effectiveness of foliar treatments of imazapyr on saltcedar (Tamarix ramosis- sima) is compromised unless the plants are left uncut for 2 years after treatment (Duncan and McDaniel 1998), we com- pared the effect of the herbicide on plants cut below the stem injection site 4, 8, and 12 months after treatment. At the first site, we selected trees in three size classes, small (4 to 8 cm [1.6 to 3.2 in] trunk diameter at knee height), medium (8 to 16 cm [3.2 to 6.4 in] diameter at knee height), and clumps with three to eight trunks branching below knee height. Only the clumps were left uncut. The emulsifiable concentrate formulation of imazapyr (Stalker [BASF Corp., Research Triangle Park, NC]; 239.7 g·ae [acid equivalent]/L; 2 lb·ae/gal) was used at this location. At the second site, we selected only medium trees (8 to 16 cm [3.2 to 6.4 ft] diam- eter at knee height). In this location, we used the water-
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