If we use the K.I.S.S. principle, then here is your formula: if your tree needs water, then water it. If your tree doesn’t need water, then don’t.
But how do you tell if your tree needs water? There are obvious signs, like wilting leaves (but that can be misleading). Better yet, check the soil moisture with your hand. For some, simply sticking a finger in the ground a few inches will do (remove mulch first). Others may need to use a garden trowel. Remember, most of a tree’s roots are in the top several inches of the soil, so you don’t need to go mining for moisture. Yep, you can use a fancy soil moisture probe too. Just make sure it’s accurate. Again, keep it simple: if it feels dry, then water it. If it feels moist, you’re probably good. If it’s wet, soggy or worse (smelly) – lay off the watering for a bit.
Another method some folks employ is using “indicator” plants, i.e. plants that show water stress sooner than others. Catalpa trees and hostas have been noted as indicator plants for water stress on trees. While not perfect, these “helpers” are a possibility. When they start to look a bit wilty, they probably need a drink and so does your tree. When planting hostas with your new tree – take care to avoid damaging the tree roots. Hostas have other benefits too. They can keep lawn mowers and weed wackers away from the trunk and are less nutrient greedy than bluegrass. Overall, they can be good little helpers, especially if mulching isn’t your thing.
One of the most common questions we get is, “How much should I water?” Unfortunately, like many other tree things, the answer is not straightforward. How much water to use is dependent on your soil, tree location, the species and age of tree, etc. So, know your site and adjust accordingly. Remember, too much water will also stress a tree. All these indicators will help you gauge your tree’s watering needs. That being said, we all like guidelines……….
Watering guidelines for newly planted trees (planted in the last 2 – 3 years) recommend about 1.5 to 2 inches of water a week. More established trees that have been in the ground for three or more years need about 1 inch of water a week. If it isn’t raining enough, then the task falls to you. And if you have fast-draining or sandy soils, you may need to provide additional water especially during a new tree’s establishment phase.
Throughout the summer, and even in the fall as temperatures drop, our trees need and use water. Evergreens (spruce, pine, etc.) can be especially stressed if they go into winter with low soil moisture. They will be more prone to needle desiccation and “winter burn” which many conifers suffered from last winter. It is appropriate to water your evergreens up until the ground freezes. As for deciduous trees, once they start to hit their normal autumn brilliance with leaf color changes, you are off the hook (slight exception – if you just planted your tree).
So, how do you get water to your trees? Water bags and buckets with a small hole(s) at the bottom work well to dispense water slowly to a new tree. The same goes for a hose on a slow trickle. Be careful to not have the water pressure running too fast, or it will just flow away from the tree before it can infiltrate the soil and get to the root system. Larger trees need water under their entire canopy. Focus on the area within six feet of the trunk and at the dripline. Sprinklers work well for getting water to this larger area. One caveat: keep the sprinkler from spraying water onto the leaves and needles of your trees. This can exacerbate fungal issues like anthracnose and Rhizosphaera needlecast.
If you don’t want to water as often, consider applying organic mulch (e.g. wood chips) around the base of your tree. The wider the mulched area, the better. There can be as much as 400x more root growth under mulch compared to under bluegrass. Mulch helps maintain moisture and moderate soil temperature among a host of other benefits. Just make sure to pull back the mulch so it isn’t touching the trunk.
Check out the Tree Owner’s Manual for more tips on watering, mulching and other tree care topics.