ARS | Publication request: Herbicides for Calendula

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: November 19, 2009
Publication Date: November 19, 2009
Citation: Forcella, F., Papiernik, S.K., Gesch, R.W. 2009. Herbicides for Calendula [abstract]. The Association for the Advancement of Industrial Crops. p. 6.

Technical Abstract: The rising need for replacements for volatile organic compounds (VOC), which are used in the manufacture of paints, plastics, pesticides, etc., has placed demands on drying oils that may not be met by current sources. The primary source is eleostearic acid, or tung oil, which is derived from the seeds of the tung oil tree, Aleurites fordii. Calendic acid is an alternative drying oil, and it is produced by seeds of calendula or pot marigold (Calendula officinalis). Few other plant-based drying oils polymerize as quickly as do eleostearic and calendic acids. Importantly, calendula grows well in temperate climates, but the tropical tung oil tree does not. The objective of this preliminary study was to identify herbicides tolerated by calendula (var. 'Carola'). Efficacious herbicides are needed to enable large-scale testing and cultivation of this new crop unencumbered by broadleaf weed infestations. (Grass weeds are controlled easily in calendula with any of several grass-specific herbicides.) Preliminary experiments in a greenhouse examined calendula injury and growth responses to 21 herbicides applied in a logarithmic dilution series. This series was equivalent to 0.01, 0.1, 0.5, 1.0, and 10.0 times normal field rates for labeled crops, as well as a check (0) treatment. Products were accompanied by adjuvants where necessary. All were applied using a specially constructed spray chamber. Approximately equal numbers of soil-applied (SA) and postemergence (POST) herbicides were tested. Herbicides that appeared promising were field-tested in small replicated plots on a research farm and a commercial farm. SA and POST products were applied and examined separately and sequentially. Three SA herbicides (metolachlor, pendimethalin, and trifluralin) were safe for use on calendula at 1.0X rates. Two POST herbicides also appeared useful. Calendula exhibited no injury symptoms with imazamethabenz at the 1.0X rate. Imazamethabenz appears to be safe even when applied following any of the three SA herbicides. The second useful POST herbicide was a tank mix of desmedipham and phenmedipham, which traditionally is used on sugar beet and is known commercially as Betamix. Betamix killed calendula if applied at the cotyledon to 3-leaf stage of growth. However, if a 1.0X rate was applied at the 4- to 5-leaf stage, older leaves were burned, but the plant recovered and subsequently grew vigorously. Despite severe leaf burning caused by Betamix, this short-term affliction to the crop more than compensates for the high level of weed control afforded by the herbicide. In conclusion, five herbicides tentatively were identified for use in calendula. Additional research must thoroughly document the safety of these herbicides as well as identify additional useful products. Nevertheless, a sufficiently broad spectrum of weed species can be controlled by these five herbicides to facilitate large-scale testing of calendula in temperate zones and, ultimately, calendic acid production and VOC replacement.