Should I use hydroponics to grow stock plants?

From a New South Wales Nurseryman.

Should I use hydroponics to grow stock plants? 
I have a nursery in which I specialise in propagating large numbers of a narrow range of native trees from cuttings. I know there are a few nurserymen who grow their stock plants in hydroponics. Would you suggest that this is a good idea? If yes, could you give me some guidance on the fundamentals.

Hydroponics could be suitable in your case. It would certainly be better suited to your situation of having a limited range of similar species, than to the more common situation of having a wide range of significantly different species. In this case, it would be difficult to effectively manage all the different species.

Hydroponics is more usually known in most of the world as soilless culture. A widely accepted definition is that it is the growing of crops in systems isolated from the soil, with or without a growing medium, where all the plants’ water and nutrient requirements are supplied by a nutrient solution. The difference to pot plants, which often use soilless potting mixes, is that hydroponics applies to cropping and uses only nutrient solutions and not slow release fertilisers.

There are two major splits between the different hydroponic systems. The first is whether they recirculate (closed system) or operate by ‘free drainage’ (open system). The second split is whether they use a growing medium to contain all the roots, or are a water-based system. Other than for rare specialised propagating systems, nursery systems would be media based. The basic requirements of a hydroponic system are that it must supply the plant roots with:

• Adequate water
• Adequate oxygen
• Adequate balanced nutrients
• Sufficient void space for the mature plant’s roots

Growing media 
The bulk of hydroponic systems use media in containers. There are a few different styles of container used, from simple boxes and pots to open topped flexible plastic bags. The most popular are lay flat ‘pillow’ bags, which have the advantage of reducing evaporation from the bag.

There are a wide range of media used. Worldwide, the most common are rockwool and cocopeat, which would be used in over 75% of the world’s hydroponic area, leaving 20% to cover all other media combined, plus under 5% using a range of different water-based systems.

In general, the media used reflect price and local availability. Apart from rockwool, there is a range of inorganic media such as perlite, sand, scoria, pumice, and gravel. Vermiculite was a media promoted in the 1950s, but is now only used in mixes. Expanded clay is used occasionally, but is mostly used in hydroculture, that is, for indoor plants. Apart from cocopeat, other organic media include peat, sawdust, bark, and a number of proprietary wood-based and artificial foam products.

Media-based systems are all dripper fed, usually with a run-off of about 30%. In open systems, this is hopefully used responsibly elsewhere, such as on another crop or pasture. In closed systems, the run-off is recovered, usually disinfected, and returned for reuse.

The first vital aspect, especially for a nurseryman beginning into hydroponics is to ensure that you use a hydroponic fertiliser. This must contain all the macro and micro nutrients needed by the plant. In particular, beware of the so called ‘complete fertilisers’, such as Aquasol, which you may currently be using. These are not complete – they are wide range soil supplements, but usually lack adequate calcium and magnesium, and include unacceptable levels of urea and ammonium.

A similar warning applies to other proprietary single part liquid fertilisers commonly used in the nursery industry. I have seen numerous cases where a single part fertiliser has been used successfully to fertigate a crop in the soil. However, when they have then used it in a hydroponic system, the result has been total disaster. Typical symptoms are that the pH falls dramatically, often to under 4. Also, there is calcium and magnesium deficiency, combined with ammonium toxicity, which kills the roots. The end result was total crop loss.

For ease of management of nutrient feeding, growers use concentrated hydroponic solutions (typically 100 times stronger than in use). These must be split into two parts, normally called ‘A’ and ‘B’. This is because at these high concentrations, calcium phosphate and calcium sulphate will precipitate and also possibly iron. Therefore, Part A normally contains the calcium nitrate and iron chelate, and everything else goes into Part B.

The essential basis of management is to monitor and control the root zone solution. Growers often get obsessed with the feed, but this is only the means to managing the root zone. With most media it is impossible to get a solution sample direct from the root zone, so the best practical sample is the run-off solution.

Management is based around sampling, measuring and recording the analysis of the feed and root zone solutions. Daily measurements should be electrical conductivity (EC), pH and the percentage of the feed, which is run off. Occasional full nutrient analysis is needed for good control, especially of the run-off solution, and especially if recirculating.

In media systems, the irrigation system must be reliable, uniform and have adequate capacity. The plants’ water demand is mainly in response to transpiration, the main driver of which is radiation. The irrigation system must be capable of replacing this water taken up by the plant, especially during the peak of the day.

Because relatively more water than nutrient is taken up, especially in summer, there is a rise in EC between feed and run-off and this needs to be managed. pH also needs to be managed, although it is not as critically important as indicated in many hydroponic books. If you use a more stable form of iron chelate, such as DTPA or EDDHA, then pH from 5.5 to 7.0 is usually OK.

One design aspect particularly impacts upon management. This is the volume of nutrient solution per plant. The lower the volume of media, the less it will cost. However, there is a compromise here. The lower the volume per plant, the faster and more severe any swings in root solution EC, temperature, nutrient imbalance, and possibly pH.

The most likely direct use of hydroponics for plant propagators is for the growing of stock plants.

Hydroponics enables the root zone to be optimised for the uptake of the nutrients the plant requires. If what you need are healthy, vigorous stock plants, hydroponics is well worth considering. Especially when compared to soil growing, better yields can be obtained with the use of significantly less water.

Some hydroponic books promote the idea of ‘total control of the plant’ – this is nonsense. The scope to manipulate the plant by adjusting the nutrient balance is very limited. However, there is some potential to influence plants by adjusting the strength of the root zone solution.

The weaker is this solution, the softer the plant. Conversely, the stronger the root zone solution, the harder the plant. This gives you significant control over the ‘hardness’ of your cuttings. Other means of manipulation are the well-known ones related to the environment, rather than hydroponics, such as temperature and day length. Where legal, plant growth regulators can be useful.

For minimum vulnerability I would recommend using a media-based system, set up with a reasonable volume per plant. At least initially, it is easier to manage if operated as free drainage. This makes nutrient management easy, because the nutrient solution is run off before it gets too far out of balance. I feel certain that you would have no trouble finding a responsible use for this nutrient rich water in your nursery. Once you have gained more experience, the system could be closed and the run-off recirculated.