Measuring evapotranspiration of hardy ornamental nursery stock: a hurdle for irrigation management

Although the land area dedicated to the production of hardy ornamental nursery stock (HONS) is relatively small, the sector places considerable demands on water supplies: production is largely in pots with limited water-holding capacity, and therefore frequent irrigation is essential. It has been shown that accurate scheduling to meet crop demand (rather than over- or under-watering) benefits quality of HONS, as well as reducing water use and run-off of nutrients and pesticides. Deficit irrigation techniques, in which plants are irrigated to replace less than 100% of the water they transpire, can further control growth and may have additional advantages, for example in reducing pests and disease. Deficit irrigation, however, requires precise scheduling to minimise the risk of excessive drying of the substrate. Numerous different species and cultivars, often at different stages in the production cycle, can be grown together on a single nursery, meaning that several different irrigation schedules need to operate at once. One option is to irrigate crops on the basis of their evapotranspiration rates. In other sectors, this entails measurement of weather conditions to calculate the evapotranspiration of a reference crop, which is then multiplied by a crop-specific factor (crop coefficient) to estimate the evapotranspiration of the crop in question. Crop coefficients, however, are not generally known for HONS. Efforts to deal with this issue, as well as alternative methods of estimating evapotranspiration using gravimetric methods or remote sensing, are reviewed in this chapter.