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Fennel (Foeniculum vulgare Mill.) is grown commercially in Tasmania for the production of a steam-distilled essential oil, which is high in trans-anethole. Often, only the generative canopy is harvested since this contains the bulk of the oil and further this oil is higher in anethole than oil from other parts of the plant. Regardless of whether the whole crop is forage harvested or the generative canopy alone is removed using a com- bine-harvester, the most efficient oil production occurs when the greatest proportion of the canopy is generative, giving maximum oil yield from a minimum of fresh weight to be processed. A trial was conducted to examine the relationship between stand density and the various yield components of fennel in order to predict the likely effect on yield of increasing stem density as the short term perennial crop matures. As for most crops, planting density and biomass yield are closely related and the optimum planting density was predicted using a mathematical model. The results suggest that an initial stand density of 10 to 12 plants/m 2 , in a square layout, would produce the greatest yield of essential oil per unit area by maximising the production of the generative canopy. This density also maximises the yield of oil relative to the weight of material to be distilled. Fennel, (Foeniculum vulgare Mill.), is grown commercially in Tasmania for produc- tion of steam distilled essential oil, which is high in trans-anethole. Although oil is pro- duced throughout the plant, about 90% of the anethole is located in the fruits (Hunault et al., 1989). The fennel canopy can be divided into two components: the vegetative canopy, consisting of the leaf and stem material and the generative canopy or flowers. Fennel, as a member of the Apiaceae, produces flowers in compound umbels, with each umbel hav- ing several whorls of umbellules. Each plant produces one terminal, primary umbel, on a central leading stem. Several secondary umbels are produced terminally on lateral branches; tertiary umbels are produced on branches aris- ing from the secondary branches and so on. Plants may produce quaternary and even fifth order umbels (Peterson, 1990). The genera- tive canopy forms the upper layer of the crop and is effectively that which is harvested by a combine harvester. In Tasmania, often only the generative canopy is harvested, thereby reducing the cost of transport to the distillation unit. Somewhat higher oil yields per unit area are achieved by forage harvesting the whole plant, but net returns are reduced by the much higher trans- port and distillation costs (Desmarest, 1978; Foldesi and Hornok, 1992; Peterson, 1990). Maximization of returns to growers is therefore achieved by channelling plant resources into the generative canopy and avoiding wasteful production of excess vegetative material. Manipulation of planting density can be used to alter the stand density of the established crop. In an earlier publication by Falzari et al (2005), stand density effects on canopy archi- tecture were discussed in relation to pollen production and pollination of this protandrous crop. Stand density also has a strong relation- ship with biomass yield. In Tasmania, fennel is planted at 10 to 12 plants/m 2 . This density was reported by Desmarest (1978) to be the optimum density for fennel crops under French growing and processing conditions, includ- ing forage harvesting and distillation of the whole crop. The trials conducted by Desmar- est (1978), showed no significant differences in total plant weight, umbel fresh weight, or yield of anethole per hectare from treatments of 6 to 24 first year plants/m 2

Optimum Stand Density for Maximum Essential Oil Yield in Commercial Fennel Crops