Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2012
Publication Date: 7/1/2012
Citation: Forcella, F., Papiernik, S.K., Gesch, R.W. 2012. Postemergence herbicides for calendula. Weed Technology. 26:566-569.
Interpretive Summary: Calendula is a new annual crop that produces a special oil in its seeds with high industrial value. The oil can be used as an environmentally-sound replacement for petroleum-based products in paints, varnishes, other coatings, and inks. Calendula oil does not volatilize when drying; instead, it oxidizes and forms a tough polymerized coating on the painted surface. A high demand for calendula oil is expected in Europe and other regions where petroleum-based products steadily are being banned. Fortunately, calendula grows well in northern states like Minnesota, and it may represent an interesting alternative crop for growers. However, because the crop is so new, crop advisors and extension educators have little information to convey to growers about weed control and acceptable herbicides. Consequently, we undertook a series of studies over three years to document which of 24 commercial herbicides can be used safely on calendula. We identified five herbicides for use in calendula. These were desmedipham + phenmedipham, imazamethabenz, metolachlor, pendimethalin, and trifluralin. These chemicals are the active ingredients found in commercial products with brand names such as Betamix, Assert, Dual, Prowl, and Treflan, respectively. (Various companies make similar products but with different brand names.) Naturally, each herbicide must be used judiciously and at specific rates to be both safe for calendula and effective for controlling weeds. Knowing that these herbicides can be used safely on calendula adds incentives for specialty oilseed companies to offer contracts to farmers to grow calendula within the upper Midwest and Northern Great Plains. For this reason the results of this research are of interest to regional growers, extension educators, crop advisors, oilseed companies and processors, and the coatings industry.
Technical Abstract: Calendula is an alternative oilseed crop whose seed oil is valued as a substitute for tung oil and a replacement for petroleum-based volatile organic compounds in paints and other coatings. Calendula is not yet grown extensively as an agronomic crop, and its tolerances to most herbicides are unknown. Therefore, we screened 24 herbicides for calendula injury. Calendula tolerated soil-applied pendimethalin and trifluralin (about 1 kg ai ha-1 each). Tolerance to soil-applied metolachlor (1 kg ai ha-1) and combinations of metolachlor plus pendimethalin was less, but still acceptable. Postemergence applications of imazamethabenz (0.4 kg ai ha-1) or desmedipham plus phenmedipham (0.4 + 0.4 kg ai ha-1) also were tolerated by calendula, but the latter tank mix caused ephemeral injury and could be applied safely only after the plants reached the 4-leaf pair stage of growth. Any of the three soil-applied herbicides could be followed by either of the two postemergence herbicides without adversely affecting calendula seed yield. Moreover, a wide array of weed species can be controlled by this suite of herbicides. These herbicide options may assist growers and crop advisors when testing this new oilseed crop on commercial farms.