By Sylvia McNeill © 2020 McNeill’s Tree Service
Proper water management is important for maximizing a tree’s potential its entire life. It is especially critical during the establishment period after transplanting. The big problem is knowing how much is enough and how much is too much. Unfortunately, there is no simple answer. Watering appropriately is as much dependent on your soil and your climate as it is on the species of plant you are watering.
Information on water requirements for a tree are often on the plant tag at the nursery. If not, a quick google search will bring up innumerable resources to provide this information.
Factors you need to know about your soil include: how fast the soil takes in water (infiltration rate), how fast it will drain (percolation rate) and how much water it holds (water holding capacity). These factors are determined by the texture of the soil (sand, silt and clay) as well as the amount of organic matter but can be confounded by compaction, common in the built environment.
Climatic factors include evapotranspiration. This is a combination of how much water is evaporating into the air from the soil and how much water is transpired from the tree itself. Humidity, temperature and wind all have a role as well.
Whereas a tree may become drought tolerant with age, young trees (remember they just had a bunch of roots cut off) need help even if they are considered a “drought tolerant” species. However, “drought tolerant” does not mean you never have to water. You will often see recommendations state “water until established” but if you live in an area with minimal rainfall, you may very well be watering every week Mother Nature doesn’t supply enough for that particular plant. The general guideline for an establishment period is one year for every diameter inch of trunk.
Some guidelines state 2 to 10 gallons of water per diameter inch of tree per week during establishment, transitioning to one inch of water per week in absence of rainfall.
Timing: You want to irrigate at a time when the water has the best chance to penetrate the soil prior to evaporating into the air. For a lot of us, that probably means early morning. If you water late in the evening and are in a warm, humid climate, this could promote fungal disease. Of course, if you are in a warm, wet and humid environment you probably aren’t augmenting irrigation and you STILL have issues with fungal diseases. At any rate, avoid watering in the heat of the day, especially if it is windy and you are using overhead sprinklers as you will lose a lot of water to evaporation.
Method: Overhead sprinkler systems are typically considered the least efficient due to the conditions I just stated in the above paragraph. Soaker hose systems delivery water in fairly precise manners but can be problematical for lateral distribution. Drip irrigation is favored by many but, I must confess, I am not a fan. It puts a measured amount in a specific area rather than full coverage of the root zone. And when it comes to trees, all too often the drip system is installed when the tree is initially planted…and never moved. (See photo below). Another problem with drip systems, or any system that is buried, is if they are not monitored and maintained, malfunctions are only noticed when the plant is stressed and/or dying.
If you are watering a garden, be it flowers or vegetables, a handheld, long-necked wand can minimize evaporation while allowing you to get precisely as much water you need, exactly where you need it.
Whatever method you choose, the entire root zone of the plant needs to be irrigated. A lawn needs the entire area watered. With trees the root zone goes far beyond the drip line. In maintaining small perennial shrubs, you may be able to get away with a drip system.
How much and how often: Water deeply and less often. This is one of those recommendations that is good most of the time. However, the texture of your soil (sand, silt and clay) will have a say in this matter. The premise for watering deeply is to encourage deep rooting of any and all your plants, including turf grass. If your soil is very sandy, you have to water more often and less at a time in order to keep the soil moist as the water will drain fast. If you have a nice loam (that ‘perfect’ combination of sand, silt and clay), you may incorporate the once a week and deeply regimen, i.e., the inch per week in absence of rainfall guideline. If you have heavy clay, you are going to water less often to allow the water time to percolate down through the soil profile.
How do you determine how much water you are applying? If you are using overhead sprinklers, the old tuna can trick is great: they have straight sides and are approximately 1″ tall. Get several (empty) cans and distribute them randomly in the area being watered. Water for 20 minutes. Measure the water in each can then add those totals together. Now divide that result by the number of cans used. Take that number, which is the average, and multiply by 3 to see how much water you would be putting down in an hour.
It is a good idea to check your soil prior to watering to be sure it needs it as well as the day after watering to see how far the water actually penetrated. Adjust your timing and amounts accordingly.