Question: Why is it recommended to plant a tree in a hole twice as wide as the root ball and the exact same depth?
— Lucas H., Las Cruces, NM
Answer: Landscape installation is hard work, but a tree is a lifetime investment, and the extra effort becomes a drop in the bucket. The old “pop and drop” method of popping the plant out of the container and dropping the root ball directly into the ground is just not good enough.
Roots tend to spiral when grown in nursery pots. These circling roots must be cut to avoid slow suffocation, tree decline, and possibly death. Before planting, be sure to loosen the root ball and cut any spiraling roots so that new root growth is trained to grow outward and you are maximizing root-soil contact.
A hole with equal depth and twice as wide as the root ball encourages lateral root growth. If you have relatively loose soil, twice the diameter of the root ball may be sufficient (e.g., a twenty-inch-wide hole for a ten-inch-wide root ball). Instead of backfilling with the soil you removed, you can loosen the existing perimeter of the hole, collapsing in the sides of the hole. If your soil is compacted, you will need to extend that hole out further. Basically, any place you want roots to grow is where you need to loosen the soil and plan to apply water. Roots need water and oxygen.
A major reason to use a hole that is the same depth of the containerized tree is the crucial area where the trunk meets the soil (sometimes called the root collar). There is a slight flare visible there. If you were to push away the soil at that spot, you might see a slight color distinction that marks the soil line. When planting a tree, make note of where that line is on the tree trunk and be sure that the final placement keeps that line at exactly the same spot.
If you dig the hole too deep, that trunk flare will be covered up after planting—this kills trees all the time. Even if you backfill some soil at the bottom of the hole to make it more shallow, that soil will eventually settle, which will cause the root collar to be buried. This is why the hole should be no deeper than the depth of the tree’s root ball. Plus, the goal is to get tree roots to grow out horizontally in the soil profile where oxygen is available and water can be applied. A typical tree develops as much as 95% of its total root mass in the top three feet of soil and 75% in the top foot alone. This means the roots extend horizontally two or more times the total height of the above-ground portion of the tree.
Frequently in landscape settings, you can find trees that have been planted too deep. In these cases, the trunk disappears straight in to the soil without any flare visible. Sometimes these are called post plantings, like a fencepost. When planted too deep like this, a tree may technically survive, but deep down it is struggling and will eventually decline. Our water is too precious to waste on dying plants.
Hopefully, we are all on the same page about planting native or adapted species. One benefit of planting such species is the water savings. Judith Phillips, a landscape designer, professor, and author in Albuquerque, calls drought-resistant plants “the camels of horticulture.” Recommended tree species and cultivars for your part of New Mexico can be found using tools listed on the NMSU Extension Horticulture blog under this week’s post: www.nmdesertblooms.blogspot.com.
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Another key benefit of native and adapted plants is that they like our soil the way it is (minus compaction—nobody likes that). For native and adapted species, fertilizer or other soil amendments, like gravel or compost, are seldom recommended when planting a tree. We want to encourage roots to grow out beyond the root ball and out into the loosened soil. Amending the soil has been shown to encourage roots to ball up in the amended portion of the soil. You can, however, add a 2- to 3-inch blanket of natural mulch as a “hug” for your new tree. Don’t hug too tight! Mulch piled up at the root collar can strangle a tree.
As with any good rule, there are caveats. So far, we are only talking about perennial plantings—plants that will stay in the landscape year after year. In the case of annual plantings like vegetables, soil amendments may significantly improve performance. And if you have clay soil, amendments may be necessary to encourage water movement through the soil profile. For the most part, though, when plating native and adapted species for perennial use, these rules hold true.
Email me your gardening questions at email@example.com or share them on social media @NMDesertBlooms!
Marisa Y. Thompson, Ph.D., is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.