My first involvement with growing blueberries in a greenhouse was some 15 years ago, when I grew a wide range of varieties at Massey University in coir slabs.
It was very much a look see, and we learnt a lot of important lessons from what was globally a very new crop for protected cultivation. I suspect that it was only the second crop of blueberries ever grown in a greenhouse, and the very first ever grown hydroponically.
Our experience clearly demonstrated the importance of netting to keep out birds – the blackbirds used to sit on the ventilators and fly into the greenhouse for breakfast when the automatic ventilators operated each morning once the fruit approached ripeness.
Our second lesson was the need to use a low pH (4.5) for the hydroponic nutrient solution to reduce the risk of iron deficiency. Finally, and somewhat surprisingly, we found that there did not appear to be a need for winter chill to ensure flowering and fruiting, even though we were using a wide range of varieties – from those developed for sub-tropical Florida to those adapted for extremely cold winters such as in upper New York State. Clearly a factor which requires further examination.
A few years later I met a Mexican grower at a conference in Arizona, and he offered to show me some of the out of season berryfruit being grown in the mountains near Guadalajara. This was my first experience of seeing blueberries being grown out of season under high tunnels, with bird-proof netting. However the plants were not grown in the soil, but using a coir mulch.
Unlike most berryfruit, blueberries can best be described as similar to ballbearings in terms of robustness, so the economics of using rain shelters (high tunnel greenhouses) must be considered a little suspect, except that harvesting in the rain can be a problem, and controlled irrigation is far superior to letting nature take over. In any case a support for bird-proof netting is an essential part of intensive blueberry production.
Although many of the original greenhouse blueberry crops were grown in soil, in recent years there has been a move towards high quality
growing media. In a recent study in the Netherlands (Table 1), growing blueberries in a well formulated growing medium demonstrated clear advantages in yield compared to growing them in soil. Study by Peter van Dyke, Blueberry consultant, Netherlands.
This (of course) poses the question of what is the ideal growing medium for hydroponic blueberries. Daltons supply a growing medium which comprises predominantly composted bark mixed with pumice and some coir, whereas in much of Australia a coir/perlite mixture tends to predominate. In the final analysis the key factors are likely to be providing the appropriate moisture holding and aeration characteristics throughout the life of the crop, which will relate very much to the stability of the organic components of the mix. Nutrition should not be a problem with hydroponics, but care must be taken to avoid the buildup of excessive salts in the medium.
Plant material is a fascinating subject, as there are now a wide range of blueberries in this country: the Driscoll varieties, the large fruited varieties from Mountain Blue Orchards in Australia, the varieties bred in New Zealand by Plant & Food Research, and the older varieties imported earlier from the United States. Only time will tell which are best adapted to New Zealand conditions. What is clear, however, is that there is a potential risk in using tissue cultured planting material to establish large commercial plantings until it has been used for growing and then conventionally propagated, because of the danger of mutations occurring in the tissue culture phase.
Perhaps the real challenge with blueberries grown in greenhouses is to develop a satisfactory pruning and training system. >
The only logical system I have seen is that used by Costa at their Atherton Highlands site in tropical Queensland, where immediately after harvest the plants are cut down to about 30cm, and then the regrowth allowed to develop to 30cm tall, when the tops of the shoots are removed. The regrowth from these shoots is again topped at about 30cm until growth ceases. It appears that fruit buds develop in the axiles of the lower leaves. How this might best operate in a temperate climate like New Zealand is not clear, and the effect could be influenced tremendously by whether we are dealing with early fruiting or late fruiting varieties. Perhaps the answer is to develop a system similar to the ‘UFO’ (upright fruiting offshoots) system used for cherries in which the plants are grown in narrowly spaced rows, with a wide spacing between plants in the row. A main stem (nearly parallel to the ground) is able to produce nearly upright fruiting branches, which are replaced regularly in a renewal system to ensure continued production of high quality fruit from young fruiting buds. Pruning of blueberries is a skilled operation, and reducing the decision making requirements is highly desirable.
There is no doubt that given the correct pH (acidity/alkalinity), along with adequate moisture and nutrients and a well aerated growing medium, blueberries will grow very vigorously, particularly in high tunnels. The question is whether to use a recirculating hydroponic system or a ‘water to waste’ system. Long term, there is no doubt in my mind that the horticultural industry will be forced into using recirculating systems for environmental reasons, but in the short term, providing the leachate can be retained and used to irrigate other crops, a non-recirculating system issimpler. Of course using a hydroponic system means that the growing medium does not require any base fertiliser. The recently developed lysimeter from Autogrow offers a means of monitoring plant water use, but it must be placed so that the plants are at the same height as the other plants in the house, otherwise it will tend to exaggerate water loss. Another way of getting a handle on water loss is to measure the conductivity of the leachate, and if this starts to exceed 2.5 it suggests that the watering regime is inadequate.
It goes without saying that pollination will be required to obtain good fruit set, and although the general unwillingness of honeybees to operate in glasshouses does not always apply in plastic clad houses, to ensure good pollination the importation of bumblebees should be a serious consideration. In some areas there may well be enough wild bumblebees locally.
▴ (Top right) Blueberries Atherton Tablelands, Queensland.
▴ (Bottom) Blueberries, Tauranga, note widely spaced rows.
Fig 1: Yield of blueberries (t/ha).
▴ (Top) Blueberries in Mexico; (Bottom) A lysimeter for blueberries.