Invasive weed control within cleared, forested sites in the inland Northwest is complicated by the susceptibility of ponderosa pine to synthetic auxin herbicide injury, used to control broadleaf weeds. Herbicide injury may lead to decreased canopy volume and variable growth patterns of ponderosa pine, which is a commercially important tree species. Herbicide injury to ponderosa pine can be decreased with dormant-season applications, a timing suited to control many weeds that may occur within ponderosa pine sites. However, spring-timed herbicide applications are needed to control other weeds, such as meadow hawkweed, and that application timing coincides with active ponderosa pine growth. In this study, we determined the level of injury to ponderosa pine resulting from spring-timed aminopyralid, clopyralid, and picloram applications beneath ponderosa pine canopies. Herbicide injury to leader and lateral candles and needle elongation was evaluated 1 and 12 mo after treatment (MAT). Low rates of aminopyralid alone (0.05 kg ae ha⁻¹ [3 fl oz ac⁻¹]) and aminopyralid clopyralid (0.05 0.10 kg ae ha⁻¹) resulted in herbicide injury ratings that did not differ from untreated trees. The high rate of aminopyralid (0.12 kg ae ha⁻¹) resulted in leader candle injury on 75% of treated trees, 5% of which were necrotic at 12 MAT. Herbicide injury was observed on 30% of lateral candles. In comparison, picloram (0.28 kg ae ha⁻¹) treatments resulted in necrosis or mortality of leader and lateral candles on 65% and 40% of trees, respectively, at 12 MAT. Results suggest that use of low rates of aminopyralid alone or in combination with low rates of clopyralid minimizes the risk of nontarget injury to ponderosa pine (> 5 yr old) while controlling hawkweed with a spring application.

Nomenclature: Aminopyralid; clopyralid; picloram; meadow hawkweed, Hieracium caespitosum Dumort; ponderosa pine; Pinus ponderosa Lawson & C. Lawson.

Interpretive Summary: Broadleaf weeds under ponderosa pine canopies are often controlled in the fall when trees are dormant but the weeds are susceptible to treatment. Meadow hawkweed, however, is not susceptible to fall herbicide treatments. Managing broadleaf weeds in the spring risks injury to ponderosa pine, thus herbicide application rates were tested to determine whether injury could be minimized. Spring applications of low rates of aminopyralid (0.05 kg ae ha⁻¹) with or without low rates of clopyralid (0.10 kg ae ha⁻¹) did minimize the risk of injury to ponderosa pine when applied below the canopy. Our results suggest that use of high aminopyralid label rates (0.12 kg ae ha⁻¹) or picloram (0.28 kg ae ha⁻¹) should be avoided when targeting invasive weeds in the spring because of likely negative effects to ponderosa pine. In particular, the results of this study should improve meadow hawkweed management decisions in the Pacific Northwest, which requires spring-timed applications but are effective at low use rates.