I apologize for all the gloom. For many people, this can be serious, and unhealthy trees are widespread. But, with a little work and know-how, learning about the watering program your trees need, all will be well. Before we can come up with a solution, we need to properly understand the problem. What you need to do is spend some time digging test holes in your lawn—they repair easily—and see exactly what you have on your property. Once we know the soil depth and its consistency, we will know how to address it. This will require a little work, but once done, it never needs to be repeated, and you will KNOW, and be able to act appropriately to keep your trees healthy.
A good way to start would be to water a section of your lawn that has trees in it. I like a big sprinkler, something that really delivers some volume, simulating a good rain. Let it get wet, water for two hours. Now let's give it a day to settle. Pick two or three random sites in the lawn, stay away from the trunks of the trees and, better yet, not underneath the upper canopy of the tree, by doing this there will be minimal root disruption. Use a full-size spade, make three or four cuts into the soil forming a circle, we want to remove a large plug of soil. Either use the shovel to lift out the whole plug or pull it out with the grass, try to keep it in one piece. Set it aside and clean out the loose soil that has fallen back into the hole. Now we get to see a mini soil profile.
Look at the fresh wall of the hole you dug. In most Calgary lawns, a blackish, somewhat sticky mix of loam and clay will be what you see. If you are really curious, you will keep digging until you see a color change; below the black layer, you will find the clay layer. It can be a tan color or even greyish. When you find the clay layer, you know exactly how deep your loam layer is.
What we are really looking for is how wet the soil is at the bottom of the hole. Perhaps prior to watering, the soil under the turf was so dry that the water you added didn't soak all the way down to the bottom of your hole. That would tell us you should have watered for a longer period. What we are trying to do here is get all the loam soil wet, as this is the soil full of your tree's roots. They will grow all the way down to the clay layer, at which point they are reluctant to go deeper. They also grow all the way up to the surface. One way to encourage deeper roots is to deep water, as I am describing, where the water is, is where the roots will be. We want to water long enough to get all the topsoil good and wet, to the point where it can't hold any more water. This is called field capacity. Beyond this point, water will run off or flow away, wasting water. The topsoil is your water reservoir; if you fill it up, you then give your trees the maximum amount of water that you can, lasting the longest time, sometimes weeks. Surface watering for short periods easily evaporates away. When the water is deeper in the soil, it is naturally conserved and will last as long as possible by being deeper. Place the soil plugs back in their holes, give them a stomp, and you're good. Turf responds well to this; when you cut their rhizome-like roots, they tend to form new growing nodes at the cuts.
Let a week go by, time, heat, sunshine, and constantly drinking trees will all do their thing; don't add additional water. Remove your plugs and inspect; how are we doing? Not bad, the surface is dryer, but not bad, the bottom of the holes are still moist, still holding water, this is good, as expected, we've learned something, replace the plugs. Let another week go by, again don't add any additional water, recheck your holes. At some point, the soil will not clump well in your hand, and you will know it's drying out. Maybe three weeks later, it is time for another long soak, which will again fill up your soil reservoir, and your trees will have the steady supply of water they love and will begin to become vigorous and trouble-free. The hole digging and repeated clump removals and inspections, only needed once, have given you an infallible tool to learn how to gauge water use on your property. From mid-May to the end of September, keep the trees well-watered. Take note of the rare heavy rainfalls, adjust your schedule accordingly. I keep a water log, which includes how much rain my garden receives. 2023's log begins like this: May 15, drought continues, dry winter, low snowpack, soil is somewhat dry. First watering of a group of large spruce for 4 hours with sprinkler. June 7, 1.5 inches of rain. This last note is important; use some straight-sided container as a rain gauge, so you really know what you did or didn't get. A good rain readjusts your watering schedule, you could gain a week or two when the rain gift is doing your watering work for you.
A few more ideas about soils and watering, and we can move on to how the tree works, the mechanics of growth, and fluid movement in your tree. If you are lucky enough to buy a new home without the finished landscaping in place, this is the best, easiest time to modify your soil zone. Once grass and trees have been planted, adding and amending soil is impossible. Measure your property and determine how many square yards of soil you need. I would add a foot of quality loam, not the cheapest stuff; it's good to go see it. The addition of a foot of good soil on top of the nominal layer from the developer will give you a nutrient-rich, major water-holding capacity garden that will seriously flourish and bloom. Many people in the city are now removing their turf or sections of it; now they can garden! Turf is a major competitor with trees for water and nutrient resources. You might like some flowers instead. In nature, in the whole closed systems I earlier described, nothing is ever taken away or cleaned up. The essential elements that were extracted from the soil remain in place, slowly returning to the soil to be used again. The clean front lawn, always with its clippings, leaves, and branches removed, creates a situation that needs fertilizers more than one where some of the "debris" is allowed to remain. Pick a style that works for you and deal with it appropriately.
