By Brian Wahl, DNR Urban Forestry Coordinator, Brian.Wahl@wisconsin.gov or 608-225-7943
A simple act, a science and an art, but bottom line – all trees need water (even in autumn)
It’s a very dry start to autumn this year. Heck, it’s been a dry second half of summer, but thanks to our lovely wet spring, we aren’t in a drought yet. However, areas of the state are “abnormally dry” according to the U.S. Drought Monitor. So be sure to get out there and water your newly planted trees! If you have the time, your established trees would appreciate a cool drink of H20 as well.
If we use the K.I.S.S. principal, then here is your formula: If your tree needs water, then water it. If your tree doesn’t need water, then don’t.
As we head into fall, don’t ignore watering, especially this year as it has been especially dry for months. Even as the temperature drops and we begin to enjoy the cool autumn season, our trees still need 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. As for deciduous trees, normally once they start to hit their autumn brilliance with leaf color changes, you can reduce your watering as the trees are using less water (unless you just planted your tree). However, this year trees are turning color early but more from dry soils than from seasonality. If you haven’t seen at least an inch of precipitation in the past week, your trees likely need a drink. It is appropriate to continue watering your trees as needed up until the ground freezes or you feel too foolish watering a tree while standing in snow.
But how do you tell if your tree needs water? There are obvious signs like wilting leaves, but it is better to check the soil moisture with your hand. For some, simply sticking a finger in the ground will do (remove mulch first). Others may need to use a garden trowel. Remember, most tree roots will be 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, to keep it simple: If it feels dry, then water it. If it feels moist, it probably doesn’t need water. If it’s wet, soggy or worse—smelly—lay off the watering for a while.
One of the most common questions urban foresters get is “How much should I water?”. Unfortunately, this, like many other tree things, is not straightforward. How much water needed is dependent on your soil, location, species and age of tree, etc. So, know your site and adjust accordingly, as too much water will stress a tree too. The above soil moisture testing techniques will help you gauge your tree’s watering needs. That being said, we all like guidelines.
Guidelines for newly planted trees (typically trees planted in the last 2 – 3 years) recommend about 1 to 2 inches of water a week. More established trees (trees in the ground for over 3 years) need about 1 inch of water a week. If it isn’t raining enough, then the task falls to you.
The “5+5 rule” is simple to remember and easier to gauge than adding an inch of water. This means every tree gets 5 gallons of water plus an additional 5 gallons per inch of diameter. For example, a 2-inch diameter tree would get 15 gallons per week–five gallons for the tree, plus 5 gallons per diameter inch, so 2*5=10+5 gives us 15 gallons total.
Don’t have the time or the staff to continue watering? Employ some help. Supply your newly planted trees with a watering device (water bags, tubes, etc.), and ask homeowners or park volunteers to fill them. Some communities send out text reminders to residents living near newly planted street trees – It’s time to water! – just don’t ask me how they do that, it’s beyond me. Social media messaging to homeowners stressing the importance of watering and explaining how to water go a long way. Adding a “water alert” or other reminder banners to your municipality’s home page can also be effective.
It’s dry. Make your trees happy and go do some watering!