Of the four tree nut crops grown in California’s Central Valley, pecans tend to emerge from dormancy and progress into bloom at the latest. Initial signs of vegetative growth are visible in early April (Fig. 1), but bloom generally proceeds through May, often exposing flowers to higher temperatures than the other tree nut crops.
Pecan Bloom and Pollination
Pecans and walnuts are in the same plant family, Juglandaceae, making their reproductive strategy very similar. Unlike February’s showy almond bloom, the pecan bloom is a more subtle event with pollination relying on wind rather than bees. They produce separate male and female flowers on the same tree, a term botanists call monoecious, meaning “one house.” To promote outcrossing, the trees employ dichogamy, a strategy where the male (Fig. 2a and 2b) and female flower parts (Fig. 2c and 2d) do not mature concurrently for a particular cultivar. As a result, growers plant at least two cultivars of pecan in each orchard to ensure the overlap of female bloom with the availability of pollen to set a crop.
In the southern San Joaquin Valley, pecan orchards are generally composed of two cultivars: Wichita and Western. In each orchard, the progression of bud break and shoot elongation varies between cultivars and with tree age (Fig. 3). The first evidence of bud break is observed when the outer scales surrounding buds are disturbed, revealing a set of inner scales that protect the vegetative buds (Fig. 1) and male flower buds, or catkins (Fig. 2a). Researchers across the pecan-growing regions of the U.S. use a uniform scale (1 to 9) to evaluate bud break progression. This allows breeders and horticulturists to assess the impacts of localized climate conditions on the phenotypic characteristics of cultivars across the country. Stage 1 indicates dormant buds and stage 2 indicates bud swell, with the outer scales split and inner scales intact (Fig. 1). By stage 3, catkins may be present (Fig. 2a), and the inner scales will have split. Stages 4 through 10 are the progressive stages of leaf burst, shoot elongation and display of preformed leaves. In the southern San Joaquin Valley, emergence from winter dormancy and early bud swell (stage 2) occurs at the onset of April, whereas leaf burst (stage 4) is observed in mid-April, with vegetative growth on Wichita preceding that of Western (Fig. 3).
The female flower buds are borne on the current season’s shoot; therefore, vegetative bud break and shoot elongation progress in advance of bloom. The female bloom of Western and Wichita overlap, and the cultivars can be differentiated by the color of the female flowers. The stigmas of Western flowers are reddish brown, while those of Wichita are greenish white (Fig. 2c and 2d). In the southern San Joaquin Valley, the female flowers mature around the beginning of May (Fig. 4). Like the rating of bud break and vegetative growth, researchers use a standardized scale to assess bloom progression across the U.S.. At stage 1, flowers have emerged; at stage 2, the female flowers are receptive for pollination. Once the female flowers progress to stage 3, stigmatic receptivity to pollen has passed (Fig. 5). During female flower receptivity, maximum air temperatures may routinely range from 80 to 90 degrees F. Western catkins mature earlier than Wichita catkins and can therefore serve as pollinizers for the earliest Wichita female flowers (Fig. 4). Similarly, Wichita catkins mature later and can serve as pollinizers for Western female flowers.
Harvest Timing and Considerations
By mid to late May, the bloom season for pecan ends in the southern San Joaquin Valley. Growers can expect a late harvest of Wichita and Western, generally from late October through early November. Wet weather, however, may push harvest into December. Pawnee, an earlier-maturing cultivar, is more frequently grown in the Sacramento Valley, where harvest may be anticipated in late September through early October.
