Water Conservation in Apple Orchards

After a month of pretty steady rains from mid-May to mid-June this spring you would think we were pretty well set up for moisture in the orchard. I used to be surprised at how quickly reserves can disappear, but not any more. Even our deepest and heaviest soils can hold only so much moisture. From field capacity, two to three weeks seems to be about the limit once we move into the heat of the summer. An acre of mature orchard can loose the better part of a centimeter of moisture per day in July and early August during peak demand. That equates to 3000 to 4000 gallon imperial through the tree and by way of soil evaporation.

An acre-inch of water is about 23,000 imperial gallons or just a little more than 100, 000 Litres. An inch of rain over 100 acres of orchard is equivalent to more than 2.2 million gallons imperial. There's no question that it's easier to watch an inch of rain come down than it is to pump the equivalent of 1 inch of water over 100 acres.

Not everyone has the option of watering. The basic tools for water management in the orchard in the absence of irrigation or with limited options include the ability to mow or control competing vegetation, crop load management, rootstock choice, summer pruning and mulches. Selective watering is also considered to be a tool by my thinking.

Mowing is one cultural practice which can help conserve moisture. A sod that is well mowed demands less water than a vigorous and actively growing sod because of a shorter root system. This same principle holds true for weeds growing in the tree row. If they are left to thrive they will remove as much water or more water than the trees themselves.

Thinning and back-up hand thinning can make the difference between success and failure for many cultivars. Varieties like Jonagold, Fuji and Goldens will not finish well if there are too many fruit on the tree. The demand for water and carbon throws off the balance. Other cultivars like Honeycrisp and Enterprise do fairly well with larger fruit to leaf ratios and appear to have relatively high yield efficiencies compared to lets say Goldens. Varieties such as McIntosh and Empire are somewhere in between.

It is generally conceded that certain rootstocks do better under dryer conditions than others do. This may also relate to the ability of certain rootstocks to tolerate various soil temperatures better than others. For example, the M9 is generally less tolerant of hot soils than MM106. The M26 is somewhere in between.

Summer pruning is a practice that will reduce whole canopy transpiration in addition to helping fruit gain their share of calcium picked up by the tree. This practice can be costly on an annual basis and some producers may opt for a more thorough dormant pruning regime and skip summer pruning.

Mulches can make the difference between a decent looking crop and one that does not make the grade for finish and volume. It can be costly to move large volumes of various organic materials like straw or spoiled hay into the orchard but the results can be comparable to supplying minimal amounts of irrigation water. The added advantage of mulching is in soil temperature regulation, which could be critical for some rootstocks.

Mulches can help level out those wild swings in soil moisture, which can contribute to bitterpit problems in cultivars like Honeycrisp or any other cultivar that is especially susceptible.

The concept of partial root zone drying is one that we have not looked at here in Ontario. In the Western U.S. and Australia, researchers claim that this technique provides benefits similar to what we would call a full irrigation program. They are using half the water resources and getting good results on trees and vines. The tree or vine is tricked into believing it's water stressed when it really isn't. As a result there's less stomatal conductance and shoot growth. Irrigation is switched from one side of the tree to the other and back. This maintains the stress signal from the roots.

The other management tool that can help conserve water is the use of selective watering. If the resource is limited then it makes sense to use it where it's going to be most beneficial - premium paying cultivars or young trees of a hot new variety and varieties that need extra size or great finish depending on market.

Newer technologies like sub-surface drip irrigation can help with water conservation. Since the lines are buried, the soil evapouration factor is largely eliminated. Water leaves the orchard ecosystem by way of transpiration only after being picked up by the tree roots or at least that's the theory. The disadvantage of subsurface systems is that you cannot readily see what's going on underground. These systems should not be installed where excessive compaction of soils is likely to occur. Growers using this type of system have to rely heavily on flow meters and pressure gauges to make sure the system is functioning according to design.