Reducing fennel stand density increases pollen production, improving potential for pollination and subsequent oil yield

Abstract

Fruit set is a key component of essential oil yield from fennel (Foeniculum vulgare Mill.) under Tasmanian conditions. Fruit set in commercial crops is often low, possibly due to incomplete pollination. Fennel flowers are strongly protandrous and a series of flowers must be produced to ensure pollination. The hypothesis tested was that decreasing stand density increases the number of lateral branches, thereby increasing the number of higher order umbels and thus increasing the overlap between the periods of pollen production and stigma receptivity. A field trial was used to examine the number of umbels of each order produced under stand densities of 4, 12, 25, 50 and 100 plants/m 2. Stand density influenced the ratio of pollen producing to pollen receptive umbels and stand densities of 50 and 100 plants/m2 showed a distinct imbalance between pollen production and stigma receptivity. The data collected supported the hypothesis and it is probable that, in commercial crops, fruit set is being reduced by a lack of synchrony between pollen production and stigma receptivity. The highest stand density tested reduced total oil production. We therefore recommend the inclusion of low stand-density strips within standard-density commercial crops.