New Zealand has a well-deserved reputation for innovation in horticulture, but is well behind other developed countries in the production of all berryfruit in greenhouses, and particularly for the major crop strawberries.
This is a surprising fact in relation to the uncertainty of New Zealand’s weather, particularly in relation to rainfall, where, even in Central Otago, rain can occur at any time of the year. New Zealand does not have a stable climate like many continents. For example Mediterranean climates have a very high probability of warm dry summers and wet winters. New Zealand’s climate can best be described as maritime. Rain and berryfruit are not a good mix, as fruit quality can be severely affected, either directly, or indirectly, by disease (such as botrytis).
Greenhouse strawberries were initially grown in the field in the soil, and then in high tunnels in the floor of the house, and more recently in what is known as the table top system—in troughs placed about 1m above the ground. Clearly as the production is above the ground some form of hydroponics is necessary, and the usual method is by means of a media based system (rockwool, coir or peat), although liquid based systems such as nutrient film technique (NFT), and even aeroponics have been used. In New Zealand some growers use a triple row system, with the central row being a further 60 cm above the two outside rows. Picking is simplified by this system, but there is no evidence to suggest that yield is affected either up or down, compared with the “traditional” two row system. The triple row system comes with a weakness in the difficulty of providing the optimum watering. Unless one is using a recirculating hydroponic system it is necessary to irrigate to ensure adequate leaching of excess fertiliser. Thus the irrigation frequency of the top row needs to be greater than for the two side rows. A similar requirement (to treat the irrigation of varieties independently), is also necessary.This is because varieties do not lose water at the same rate - for example vigorous/leafy varieties will transpire more water than smaller varieties, and thus build up salinity (a high conductivity) in the solid medium. Obviously not so important for NFT systems, where the nutrient solution is recirculated.
In relation to media conductivity, there is an interesting report from the UK which suggests that media conductivity not only effects yield, but also that strawberry varieties perform differently depending on media conductivity.
Of course, moving into protective cultivation offers the opportunity to consider the potential of using integrated pest management to minimise the use of pesticides. With a “roof” over the crop, and the fruit held well above the ground, the risk of botrytis fruit rots is minimised, and the main problem to overcome are pests such as spider mite, cyclamen mite, thrips, aphis, white fly and, on occasions, caterpillars. Biological control of caterpillars is straightforward with BT (Bacillus thuringensis). White fly is controlled by Encarsia formosa:
spider mite by Phtoseiulus pesimilis; and thrips by Neoseiulus cucumeris, which also exerts some control over cyclamen mite, a major problem with the variety Albion. Aphis has a range of predators, including Aphidius,
and black ladybirds. This is not to suggest that pesticides are not necessary, in fact it is a good idea to ensure that planting material is kept as free as possible from pests with regular use of appropriate pesticides. It is only during the fruiting period that some form of integrated pest management might be considered as a potentially valuable production strategy.