Hydroponic fruit tree production

Growing potential?

By John Fitzsimmons

Awareness of growing plants hydroponically is nowadays very high. A good understanding of this approach is, perhaps, not so high. ‘Hydro’ is commonly associated with food crops such as tomatoes, capsicum, lettuce and herbs (culinary, medicinal and ‘recreational’) and with some ornamentals and in floriculture (e.g. roses). But would it, or does it, work with perennial crops like fruit trees?

For certain food and floriculture crops, hydroponic production has become the dominant production method, especially in combination with protective structures such as greenhouses. Thus, both the root zones and the plants’ terrestrial parts are given appropriate input and care. Hydro has also been successfully used in a broader range of crops than perhaps first thought – think soft fruits such as blueberries, strawberries, lavender, and celery. Technically, almost anything can be grown in hydro,  even root crops. But that ‘possible’ list shrinks quickly when true commercial criteria and ultimate viability are imposed. Home gardeners and hydro hobbyists should not be discouraged from discovering what might be ‘possible’.

But what about larger shrubs or fruit trees? They are also plants, so, technically, hydro production should be at least possible. One of the biggest challenges is mechanical – the size, weight, rigidity (or otherwise) of canopies, and the commensurate balance of rootzone needed to support overall plant viability and/or foliage and fruit production. I have seen small eucalypts grown semi-hydroponically in European greenhouses, but that was producing foliage as fodder for marsupials in zoos – a bit niche. It’s not necessarily commercial and certainly low volume.

Without ending up in a full re-introduction to hydroponics, let’s remember two of the main categories are ‘recirculating’ or ‘re-use’ systems, and ‘run-to-waste’ or ‘open’ systems. Both are self-explanatory, with the former widely used in various sub-formats. The latter largely fell out of favour due to environmental impacts such as the adverse contributions of (surplus) nutrients into soil profiles and catchments, apart from other impacts on neighbours and the obvious economic wastefulness.

However, an evolution of the open hydro idea, Open Field Hydroponics (OFH), did emerge 20-30 years ago and has carved a small but valuable niche in fruit production, specifically in citrus, although there has been research done with other genera too, including persimmons, peaches and nectarines. Quite recently, a large commercial citrus orchard in Australia came on the market, and its use of OFH was utilised to promote the sale – proof of some legitimacy.

However, the OFH idea has been taken further with researchers demonstrating the possibilities of OFSS (Open Field Soilless System). This idea combines ‘normal’ field growing with supplementary supply of water and nutrients to separated sections of the root system via a hydroponic supply. Amazing potential or what?

OFH and OFSS grew out of research into improved Water Use Efficiency (WUE) and Nutrient Use Efficiency (NUE).­ These criteria have become increasingly important due to increasing population, resource limitations (including suitable land, water, and plant nutrients), and environmental concerns.

In broad terms, it was found that tree growth and productivity could be improved by using high-frequency, low-volume irrigation, focusing on supplying the tree’s daily water requirements and moving away from the idea of using the soil in the root zone as a reservoir.

Citrus and stone fruit orchards in modern times were watered by overhead irrigation. Then, under-tree low-volume systems came into vogue. With OFH, there is more attention on drip systems, with emitters at close intervals to ensure the ‘onions’ of irrigation water distribution in the soil profile effectively meet, to ensure a thorough supply to the tree’s roots. Water supply is limited to avoid over-application and subsequent leaching of nutrients into water tables and catchments. This is in addition to using Deficit Irrigation (controlled minimisation) methods to improve fruit quality.

Thus, by objective management of fertigation, it was shown the tree’s rootzone can be physically limited in-soil while being optimally fed and watered without adverse environmental outcomes. The tree also remains anchored and supported by the soil.

Now that idea has been taken a step further to OFSS.

Using soilless and hydroponic concepts, this is perhaps better described as ‘near soilless’ production.

In OFSS, part of the tree’s root system is allowed to establish naturally, while a larger proportion is influenced to be accommodated in parallel grow bags, which are serviced by hydroponic fertigation systems. The grow bags can begin sitting on the soil surface next to young trees and be relocated below ground as trees mature.

It’s crazy, but the idea reminds me very much of a massive tree root invasion of stormwater pipes, only intentionally.

The tree establishes in-ground and remains effectively anchored and supported. Being nurtured by both field soil and one or more controlled and contained hydroponic ‘zones’, fluctuations in water and nutrient availability are less dramatic and, to some degree, easier to control. 

Much research has been done into suitable growing media, pH, nutrient solution content and strength (EC), and fertigation programming (volume and interval) relative to the tree’s growth stage in OFSS situations.

It appears that this system is best implemented using drip irrigation with emitters closely spaced and suitable daily drip or pulse fertigation to satisfy the tree’s transpiration and evaporation needs. Nevertheless, plenty of homework, more focused research, and finessing are needed with the genus you are interested in to improve the chances of success beyond current research outcomes.

Being somewhat of a hybrid system means OFSS still has a lot of local and site variables to be considered, but the potential appears to be significant. In horticulture and landscape, for example, consider the scope for sustaining healthy and productive fruit trees in limited rootzone situations or where there are challenges for below-ground infrastructure (pipes, cables, or structures).

OFP has certainly been commercialised in several recognised production regions around the world. Its use with perennial trees seems to be accepted. It has been suggested that OHSS might be more appropriate for deciduous trees adapted to longer seasonal dry spells.

These ideas can improve water and nutrient use efficiency, and open up growing potential on otherwise unsuitable soils where environmental risks should be reduced.

On the other hand, implementation costs can be high, and there is a clear need for high-quality expertise and system management. Access to inspect or maintain drippers in grow bags (above or below-ground) may be compromised, and depending on the material and quality, the grow bags may have a limited lifespan (3 years?). Growing media in the bags may also ‘slump’ over time, affecting the system’s functionality.

It has been suggested that higher tree densities and complementary use of dwarfing rootstocks could enhance the chances of success.

In summary, OFH and OFSS have been shown to have potential and have entered the commercialisation phase. Opportunities and drivers exist. Let’s review the idea down the track.