Researchers conducted a study near Matlock, Washington investigating the potential of logging debris and herbicide combinations to inhibit germination and development of Scotch broom (Cytisus scoparius) seedlings. The study site was a mature Douglas-fir forest that was scheduled for harvest. The forest understory included occasional Scotch broom plants that invaded from a previous disturbance, indicating the likely presence of soil-stored seed.
Timber was harvested from the study area in November and December 2011, and debris plots established. Study plots were arranged as a randomized complete block, split-plot design with six replications. Main plots included three soil surface treatments (light debris, heavy debris, and machine trails), with debris depths averaging about 6.5, 12.5, and 6 inches respectively (Figure 1).
Herbicide treatments were applied in August 2012 with backpack sprayers. Treatments included Garlon® 4 Ultra specialty herbicide at 2 pounds (lbs) acid equivalent per acre (ae/A) and Milestone® specialty herbicide at about 7 fluid ounces per acre (fl oz/A) applied either alone or in combination. Douglas-fir seedlings were planted in February 2013. Scotch broom regeneration was measured within 0.1 m2 frames.
Results of the debris study showed that density of current-year seedlings of Scotch broom doubled from June to July, 2012 (1,800 seedlings per hectare (ha) to 3,800 seedlings/ha respectively). In July, broom density was lower in heavy debris than on machine trails, but it did not differ significantly from that in light debris(Figure 2A).
Results of the herbicide treatments showed that the combination of Garlon 4 Ultra and Milestone reduced broom density by about 90 percent seven weeks after treatment (Figure 2B) compared to non-treated plots. Although the herbicide combination provided the best control of Scotch broom, this treatment did not differ significantly from plots treated with either Garlon 4 Ultra or Milestone alone.
In summary, these results suggest that either heavy debris (12 inches or greater) or application of Garlon 4 Ultra and Milestone combined was the most effective at reducing broom seedling density. Both of these treatments reduced density of first-year seedlings by about 90 percent. Debris treatments inhibited broom germination by altering soil and light environments.
NOTE: While Milestone is not registered for use in forestry in western states, Opensight® specialty herbicide (as Special Local Needs (SLN) label in OR, ID, and WA) and Capstone® specialty herbicide (federal label) are registered for use on forest sites.
Active ingredients for herbicide products mentioned in this article: Milestone (aminopyralid), Garlon 4 Ultra (triclopyr), Opensight (aminopyralid plus metsulfuron), and Capstone ( aminopyralid plus triclopyr).
® Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Label precautions apply to forage treated with Milestone and to manure from animals that have consumed treated forage within the last three days. Consult the label for full details. State restrictions on the sale and use of Garlon 4 Ultra apply. Consult the label before purchase or use for full details. Always read and follow label directions.