Response of Branching and Nonbranching Cut Flower Cultivars of Sunflower to Pinching and Planting Density

Abstract

Pinching sunflower (Helianthus annuus) grown for cut flowers in the vegetative stage increases stem yield at least 3-fold. Four experiments were conducted to further investigate the cultivar response differences to pinching, to explore the response to planting density, and to examine the effect of the time of pinching. In the first two experiments, two branching and one nonbranching cultivar in 2011, and three branching cultivars in 2012, were grown at 6 × 6-, 9 × 9-, and 12 × 12-inch spacing and pinched at the six-leaf stage. Pinching led to stem yield increases, but reduced flower size and stem length. Combining closer spacing with pinching further decreased flower size, increased the number of stems, and the gross value of stems per unit area. At highest densities flowers were too small to be saleable. The inverse relationship of flower size to stem numbers per unit area held true across all cultivars in both years. The branching cultivar Starburst Lemon Aura was the most productive of the cultivars tested when pinched, yielding 15,744 stems/1000 ft2 at the closest spacing. In 2013 and 2015, delaying the time of pinching from the 6-leaf to the 10-leaf stage reduced stem production among three nonbranching cultivars from 2.8 to 1.8 stems/plant, compared with an increase from 4.2 to 5.1 stems/plant for the branching cultivar Goldrush. The difference in pinching response among cultivars in 2013 and 2015 may have been related to relative severity of septoria leaf blight (Septoria helianthi) that infected the lower leaves. The results of the four experiments confirmed the efficacy of pinching on productivity and value of sunflowers, with best results obtained when pinching was done at the six-leaf stage.