Plant Health in the HEAT: Water Those Trees!

July-August 2012

Many of the southern Ontario conservation authorities declared a Level I Low Water Response in their watersheds back in May (in May!). I’ve never seen anything like it. Some of these watersheds are very close to declaring a Level II. The average precipitation across the Hamilton Conservation Authority watershed was between 40-60% of normal for this past spring. So to get on topic, if you think that poor little tree doesn’t deserve some irrigation, think again!

Most newly planted trees require irrigation, especially in the first year or two of planting. They have a very small root system and the water use demands of the canopy will not be met without a little help from us. It’s been extremely dry since early spring, which means that our soils don’t have the moisture reserves they normally would coming into the summer season. 

When soils are this dry, it takes a long time to wet them again. Irrigation water will actually bead off dry soils and mulch. That’s where a long, slow, deep soak using low volume and low pressure comes in. This slow delivery of water will be required for the moisture to actually make it down into the root zone. This can be achieved by slowly applying an inch or so of water to the soil every 7-10 days during hot, dry conditions. Researchers at Michigan State University came up with the following: 

Crown Spread (m) : Water Required (L)
2 : 75
2.5 : 115
3 : 190
3.5 : 265
5 : 475
6 : 750 

Many homeowners have automated irrigation systems installed in an effort to reduce the stress on their landscape plants. In most cases, the irrigation systems have been designed to deliver water efficiently to the root zone and can be deployed after sunrise to facilitate leaf drying by mid-morning. Any time we spray water on leaves, those leaves can stay wet for hours and that period of time is called the “leaf wetness period.” There are several different kinds of fungi that can infect leaves after only a 6-8 hour leaf wetness period, some 8-12 hours. If overhead irrigation is used in the evening, you can imagine how easy it would be to create ideal conditions for disease.  

Recently I had an arborist come to me with disease issues on several different species of plants on one client’s property. Now it’s been hot and dry, how is this possible? As it turned out, they had an automatic irrigation system and they really liked to use it. When asked how often they irrigated, the arborist said: “Well they used to irrigate every day but now they’ve backed it off to every other day.” I actually hear this lot (and continue to find it quite surprising considering the amount of public education on water conservation). So now that we know what the problem is, the challenge is to educate the client without insulting them.  

Sprinkler-type emitters are still very popular as they can deliver large amounts of water over large areas. Unfortunately, they are not very efficient because a significant percentage of the water droplets can be lost to wind and evaporation before they even hit the ground. In some cases, low volume irrigation (e.g. drip) systems can be an effective alternative to deliver water much more efficiently to the root zone while keeping water off of the leaves. 

Anything you can do to design the system and landscape to minimize leaf wetness periods may result in a much healthier, cleaner looking canopy. I have seen this principle employed very successfully in nursery production. Several times I have had growers tell me that their disease issue goes away after they switch the irrigation cycle to deploy after sunrise (and completely avoid irrigation at other times of the day). They usually have a goofy smile on their face when they are telling me this because I have been known to “nag” about leaf wetness.      

Each time I pick up my daughter from her after-school babysitter’s house, I usually stop and have a look around at their gardens. This family has a beautiful home and they put a real effort into keeping the gardens and lawn looking great. This spring they planted a B&B Colorado spruce out front. They said it took about 8 hours with a pickaxe but they finally dug the hole! I realized that the tree never actually leafed out this spring and this week I noticed that telltale, dull grey hue that spruce get when they are on their way out of this world. It was time to tell them – and they weren’t happy. What happened to the tree? I think if you’ve been reading my columns all these years, you should be able to have some pretty good guesses lined up!  

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