Goldilocks was an unwelcome visitor to the three bears’ cottage, walking in unannounced without being asked to enter the house. She helped herself to the porridge, chairs and beds, finding that one was too much, one was too little and one was just right in each case. It’s not clear from the story what the bears thought of Goldilocks, except that they growled and exclaimed a bit. Goldilocks seemed to be scared of the bears and ran off without any verbal exchange between them, so we’ll probably never know, unless there’s a “Three Bears” sequel.
Even though this tale was a bit unsettled for the future, it’s still instructive for watering trees and keeping them in good health. Taking the cue from Goldilocks, the theme of “just right” should be the goal of every tree grower and tree care provider.
Moist, wet, dry
So, what is “just right”? Is it moist, wet or dry? Conventional wisdom, at least for most homeowners, would be that trees like to have wet soils. In fact, it’s a common nursery or garden center complaint that newly-sold trees are killed by overwatering by customers, who dump bushel baskets full of water on the new specimen every day or let the hose run for six hours or more every time they look outdoors to admire their new purchase. It just makes them feel good to do it.
When it comes to being wet, there are trees that are tolerant of wet soils, but few like to grow in wet soils. Why? Trees need oxygen in the root system to function and grow well. Trees such as swamp white oak, zelkova, willow, dawn redwood, river birch, serviceberry, persimmon, sweetgum, holly, magnolia, sycamore, black gum and alder are known to be tolerant of wet soils, especially ones that are intermittently wet.
The opposite is true for dry sites, where due to lack of soil moisture, tree roots simply dry excessively and wither and die, causing major stress to trees. Just as for wet sites, a couple of species that are super resistant to overly dry soils stand out, such as hackberry and Eastern red cedar. But again, the majority of trees that are considered to be drought tolerant are just that – tolerant. Such species include hedge maple, hornbeam, yellowwood, filbert, hawthorn, ginkgo, golden raintree, bur oak, chestnut oak, sassafras, Japanese pagoda tree, golden raintree and Siberian elm.
In addition to the individual preferences of specific trees, there are overarching factors that complicate watering:
- Watering lawns: In many landscapes, trees are growing smack dab in the middle of a lawn. Generally, trees require less water than turfgrasses, such as Kentucky bluegrass, perennial ryegrass, hybrid bermudagrass and St. Augustine. As a result, they tend to be overwatered, because they share the same root zone. Others, such as bahiagrass and tall fescue, can usually get by with the same amount of water required for the majority of trees. Overall, the best solution is to design landscapes that separate turf from trees. In mature landscapes, it’s usually not feasible to do so. But some of the turfgrass can be replaced with ground covers that require less moisture. This is a good option in that they often are shade adapted as well, unlike turfgrasses. Lamium, periwinkle, goutweed and Japanese spurge are good groundcover choices for shade.
- Poorly drained soils: In soils that are poorly drained due to clay, compaction, wear or heavy traffic patterns, watering becomes difficult. These soils generally absorb water slowly and then hold it for a longer time as opposed to soils that are better drained. Probing soils with a long screwdriver will help determine the moisture content.
- Excessively well-drained soils: Usually comprised of a heavy sand or silt content, these soils have the opposite problem, in they lack the capacity to hold water for a sufficient time for the plants to utilize it. On these sites, more frequent applications with lesser volumes are required to keep soils at the proper moisture level. Probing is useful to get a handle on the level of moisture between the soil particles.
- Small root zones: Trees planted in small root zones, i.e., tree pits, hell strips, parking lot islands. etc., are subject to difficulties in watering, as well. Because of the limited surface area of the small space, it’s hard to keep enough water on the soil without having it run off the site into the street or other impervious surface. In such situations, trees often dry out and become stunted.
- Slopes: Speaking of runoff, slopes are the quintessential runoff site. The key here is that they cause the precipitation or irrigation rate to exceed the infiltration rate. The frequent result of watering trees on slopes is that the lower half of the root ball is adequately moistened, but the upper remains dry.
- Amended sites: Also sometimes called large pots for trees, planting sites that have been heavily amended with compost, manure, peat moss or rotting hay to loosen clay soils or enrich sandy soils often create unintended consequences for tree survival. Trees planted in these situations usually grow quite well for the first few years. Then, as the roots reach the sides of the original planting hole, they’re restricted by the nonamended sides and aren’t able to penetrate them, and they turn back toward the center of the hole.
- The result of the amendment is either an inadequate root zone or stem girdling roots (or both).
Keep it moist
Where the goal is to find the sweet spot and keep the soil moist, there are three main tools or influencers – the devices, the methods and mulch.
Many water emitters on the market today do a good job of applying water in the right place, at the right rate and at the right time. Yet, some are simply better suited for vegetable production or lawn maintenance. Finding the right one for the client’s tree is an issue of matching up the tree with the landscape setting. As mentioned earlier, this is much easier if the tree and turf are separated so that each can be watered on a different zone, according to its own needs. Overall, the devices that place water over the tree roots without spraying water high into the air are considered highly desirable. Such products include drip emitters, microspray heads, water soaker bags and soaker hoses.
Using the right tool is good; using it correctly is better. Generally, watering trees should be done in a similar fashion to cooking a piece of meat with lots of tendons or collagen in it (roasts, brisket, ribs) – low and slow. Low so that the water can stay where it needs to be without flying away via drift, and slow so that it can soak in before running off. Periodic checking is recommended for both endeavors – checking temperature for cooking brisket and checking the moisture content of the soil in the root zone for trees. Once the soil is sufficiently moist after watering, it’s helpful to keep it there for a couple of weeks or so. There’s nothing better for doing so than applying 2 inches of a coarse, loose wood chip mulch.
Going back to the Goldilocks analogy, the aim is this: not too much, not too little, but just right. Unfortunately, an increasingly common sight in DIY landscapes is a “mulch volcano,” where mulch is mounded 12 to 18 inches high around the tree base. There are several negatives associated with this practice, including increased potential for rodent damage in winter, increased runoff of irrigation/rain water and keeping the bole of the tree overly moist, which often results in decay or Armillaria root rot.