Comparing E-Z-Ject application of Roundup with foliar application of Krenite in the control of aspen (Populus tremuloides Michx.) in tallgrass prairie

Summary

Aspen Populus tremuloides may sometimes colonise floristically-rich tallgrass prairies. Once established it spreads clonally by underground stems and if allowed to grow tall, shading drastically reduces the diversity of native prairie vegetation. Burning is a traditional method of managing these grasslands and whilst this kills aspen seedlings, once established, larger shoots and saplings are fire resistant and their growth seems to be encouraged by burning. Formerly aspen colonisation was kept in check by grazing animals and the spread of aspen is perhaps a consequence of the extripation or severe reduction in numbers of grazers such as elk Cervus canadensis, white-tailed deer Odocoileus viginianus and beaver Castor canadensis. In order to conserve an area of high diversity wet-mesic prairie in Wisconsin, aspen control efforts focussed on cutting and herbicide application, and a study was undertaken to compare techniques in terms of effectiveness, cost efficiency and impact on non-target species.

Study area and treatments: The study was undertaken at Faville Prairie State Natural Area in southeast Wisconsin, USA. As well as cost-effectiveness of aspen control, a priority was to minimize damage to other prairie vegetation. Therefore methods were selected that:

i) required no vehicles on site;
ii) had low risk of herbicide damage to non-target species;
iii) used herbicides with a short soil residence time;
iv) did not leave burn piles (i.e. piles of vegetation that would would burn strongly (thus potentially killing prairie vegetation, mychorrhizal fungi and sterilizing the soil).

It was decided not to undertake girdling (bark-ringing) or winter stump treatment with Garlon-4 (a herbicide), although managers have reported excellent success with these two treatments. Girdling, especially of small diameter aspen, is very time consuming and herbicide stump treatment (even if undertaken in winter when many plants have naturally died back) may occasionally result in damage to non target vegetation.

A literature survey and results from earlier trials at the University of Wisconsin-Madison Arboretum indicated two control techniques that met the required criteria:

i) injecting aspen trunks with Roundup (active ingredient (a.i.) glyphosate) using an E-Z-Ject lance;
ii) cutting the clones followed by a foliar application of Krenite (a.i. fosamine ammonium) to re-growth.

Aspen clones: Once established in grassland, aspen spreads clonally by underground stems. The profile of these clones is typically bell-shaped i.e. the tallest trees are in the centre and height quickly decreases outwards. The perimeter of a clone is a skirt of small stems hidden amongst other vegetation. Taking this into consideration, in 1998, 20 discrete aspen clones were identified on the prairie. The perimeter of each was marked and live aspen stems counted, placing each within one of six size classes: >1.2 m tall; <1.2 m tall, >0-13 mm diameter at breast height (dbh); <1.2 m tall, 13-25 mm dbh; <1.2 m tall 25-50 mm dbh and <1.2 m tall >50 mm dbh. There were only two trees with a dbh >10 cm.

The 20 clones occupied areas ranging from 120 to 900 m². Stem counts numbered from 21 to 1,765 (average 473) and most (62%) were less than 1.2 m tall. Ten replicates of each of the two treatments were randomly assigned to the clones. During the treatment phase for each clone the following were recorded:

i) number of hours spent;
ii) quantity of materials used;
iii) comments on level of application difficulty.


E-Z-Ject method: Two E-Z-Ject lances were used to inject capsules (22 calibre cartridges) filled with a paste of Roundup (0.15 g glyphosate/capsule) into the stems. The 10 clones were treated in September 1998, the assumption being that application in late summer maximises transport of glyphosate into below-ground tissues. Capsule delivery prevents herbicide drift which would be a problem with spraying or stump painting with Roundup (as it is a broad-spectrum herbicide). The manufacturer's recommended dose of 1 capsule/each 5 cm diameter up to 20 cm diameter, and 1 capsule/each additional 3 cm diameter increase for larger stems, was followed.

From previous experience lance operators working alone become disorientated resulting in some stems being left untreated. Therefore, operators worked with an assistant who acted as a guide and marked injected stems with yellow chalk.

Krenite method: The aspens were cut, returning once new growth had sprouted and then applying a foliar application of Krenite. In August 1998, using brush cutters and chain saws all aspen sprouts and trees of the selected clones were cut down. Cut material was carried and scattered beyond the perimeter of the clone to prevent entanglement where herbicide would later be applied. Scattering also prevented accumulation of debris into burn piles.

Within a few days of cutting, new aspen shoots were growing and applying Krenite to these initial sprouts may have increased mortality. However, staff were not available and treatment was spread over the subsequent two years.

By spring 1999, there was a dense growth of young aspen stems and over the midsummer period using backpack sprayers, Krenite was applied. The herbicide solution was a 1:8 mix of Krenite and water, equivalent to a 3 % fosamine ammonium mixture. To each two gallons of this mixture, 1 oz of Cidecick (a spreader sticker) and 1 oz of Blaze On Blue (a dye to indicate which foliage had been sprayed) was added.

Krenite is effective in the control of many trees and shrubs but should have little effect on most herbaceous plants - one pest species, leafy spurge Euphorbia esula, is a notable exception. The effects of spraying are not generally evident until the following spring when leaf buds fail to open.

Monitoring: Following treatments, monitoring was undertaken to look for evidence of any effects upon both the aspen and non-target plants.

Effectiveness of treatments: Overall, the Krenite approach was the more effective of the two treatments at decreasing aspen of all sizes (Table 1, attached). In 2000, the number of stems over 1.2 m tall were reduced from 1,462 to 46 (97% decrease) but this was not surprising as the aspen had been cut down in 1998. The 46 stems over 1.2 m were all in the smallest diameter size class (<12 mm dbh). The number of live stems over 1.2 m tall in the E-Z-Ject treated clones was reduced from 2,538 to 493 (an 81% decrease) with some living trees left in all size classes. It was found that there was a decrease of effectiveness with increasing stem diameter suggesting that the manufacture's recommended dose should be increased in larger stems.

Cost: Another advantage of the Krenite method was the lower cost of chemicals ($26/1,000 trees) as opposed to E-Z-Ject method ($164/1,000 trees > 25 mm diameter; stems < about 25 mm in diameter were too small to be injected). Additionally Krenite application required no additional purchase of equipment whilst the the two E-Z-Ject lances cost $487 each.

Man-hours: On the downside however, the Krenite method required over 3-times as many man-hours and was physically much more demanding with 1,000 trees taking an estimated 32.8 h to treat as opposed to about 10.4 h/1,000 trees for the E-Z-Ject method. The E-Z-Ject method also offered the flexibility to switch periodically between operators and guides but staff reported frustration when capsules jammed in the lance or when capsules did not lodge properly into stems.

Damage to non-target species: Each treated area was inspected carefully for herbicide damage to non-target species but non was found. There was considerably more trampling damage in the Krenite treated areas but it was doubted that this would cause long-term damage.

Bird seed dispersal: In the E-Z-Ject clones, as of October 2002, living and most dead stems were still standing and an unexpected problem was discovered. These stems provded attractive for birds to pearch upon and under them several bird-dispersed potential pest woody plants e.g. common buckthorn Rhamnus catharcticum and blackberry Rubus allegheniensis were found. In the Krenite trated area where few perches were available, these plants were not present.

Re-establishment of plants and re-injection: By the summer of 2002, prairie grasses had re-established across much of the previously heavily shaded areas. At this time the remaining living trees in the E-Z-Ject clones were re-injected.

Conclusions: Both methods have a role in aspen control. The Krenite method was more effective at reducing stem numbers, did not leave standing trees and cost less in terms of materials. The E-Z-Ject method was easier to employ and took much less time to treat a similar number of trees. It is considered that in many situations using the E-Z-Ject method on larger stems and the Krenite method on stems short enough for easy foliar application would be the best use of resources.


Note: If using or referring to this published study, please read and quote the original paper.