Anyone who plants or cares for woody plants eventually hears the term “root flare” (or root crown). It’s easy to describe a root flare (it’s the region where stem or trunk morphs into roots). What’s sometimes difficult or even impossible is finding it in improperly planted trees and shrubs.
One of the primary causes of tree and shrub failure is improper planting depth. This is not a problem with bare-root plants, as you can easily see the region of transition. During planting you should make sure that the root flare is at grade, so that the roots are underground and the stem/trunk is above ground. The only mistake you can make with bare-root plants is to plant them upside down.
The problem really started with the advent of containerized and balled-in-burlap (B&B) plants. This technology is less than 100 years old, and before it existed everything was either planted from seed or from bare-root stock. It’s possible to use containers and B&B properly for temporarily housing trees and shrubs, but increasingly automated production methods with unskilled workers and undereducated supervisors means increasing numbers of poorly planted woody plants entering the retail market.
I’ve written earlier posts about how to select plants at the nursery. As you’ll note, finding the root flare can often be impossible without removing container media or B&B burlap. Because so many people are unaware of the problem or unwilling to disturb the root ball, these plants are then installed with the root flare still buried.
Why does it matter if part of the trunk is underground? For some species, it really doesn’t matter. Wetland species, for instance, can tolerate low soil oxygen levels and submerged trunks. But most of us are not planting wetland species, and many ornamentals are not tolerant of this treatment. Roots that are buried too deeply don’t receive enough oxygen to survive, and the plants respond by trying to create a new root system. These adventitious roots are unable to supply enough water to the growing crown, however, meaning shrubs and trees suffer chronic drought stress when the rate of evaporation exceeds the ability of these substandard root systems to supply water.
There are other problems, too. Stem and trunk tissues of non-wetland species are not adapted to being buried. The excessive moisture and lack of oxygen contribute to the attack of opportunistic pests and diseases, both of which can cause irreversible damage and eventual death. You can even see this happening to plants in the nursery.
Finally, consider this landscape evidence of the impact of buried root flares. These magnolias are all planted on the campus at Princeton University. The one of the left is significantly smaller than the other three. A close up of the trunks explains why.
If you have newly planted trees that look more like telephone poles than trees, the best thing you can do is dig them up and plant them correctly.
Dr. Linda: looking at the pic of the lilac planted too deeply. i’ve transplanted some lilac suckers. they were less than 1 year old, hence very small. how does one figure out a trunk flare in that scenario?
Corey, you should be able to see where the roots emerge. Assume that the uppermost root represents the crown and plant accordingly. In the photo shown, there are multiple trunks of one plant, but you can’t see the place where they all connect. That’s the flare.
Very useful information! The US Tree Care Standard sorted the terminology:
82.10 flare (trunk flare, root flare): 1. The area at the base of the plant’s stem or trunk where the stem or trunk broadens to form roots
82.20 root collar: The transition zone between the flare and the root system (where the buttress roots divide)
When this distinction is clear, growers and landscapers can more easily understand that the bottom of the flare, the root collar, should always be visible.
Linda, thank you for this post and your posts about using mulch to kill grass instead of cardboard/paper. I got your message about deep mulch, but the area I’m trying to convert from lawn to garden bed contains a mature apple tree (MD z7). The grass goes right up to the trunk, which appears to go right into the grass without much flare. Will deep mulch harm the roots of my tree, or is it ok because it’s not being planted? I assume it’s about 20 years old and seems healthy. Last year a bumper crop of apples, this year no flowers but many healthy leaves and our neighbor’s apple tree a block away hasn’t flowered either. Thank you!
Deep layers of coarse wood chips will not harm your tree. It’s much, much better than all that grass competing for water, nutrients and space!
Thanks for the info & useful pictures! We’re in Zone 8a Southeast & having air spade work done on our too-deeply buried trees (a large, mature oak, two teen dogwoods, a couple of mature magnolias & a group of mature, previously topped, Leland cypresses) now. I’m having difficulty getting specifics on aftercare, like guidelines for how much & what methods for watering, whether or now tree wells are needed, science-based (hopefully definitive) mulching type & placement, etc. The arborist is not as specific as I would like- esp about how much watering is needed for best results- & other sources contradict some of his recommendations, especially about what & how much soil/mulch/other material should go back in the flare zone. My Internet searches aren’t turning up the info. Please can you give some of this and/or direct me to another resource? Thanks again!
There really is not a pat answer for how much water – you will need to monitor soil moisture, probably with your fingers, to assess this. You don’t want it soggy and you don’t want it dry. It should leave a residue on your hands and feel moist to the touch.
Mulch deeply with arborist wood chips. Maintain a minimum of 4-6″ to reduce weeds and enhance root establishment. Don’t skimp on this step. The chips can cover the flare as trees are adapted to a coarse woody debris layer.
You can use this university fact sheet for information on wood chips: http://cru.cahe.wsu.edu/CEPublications/FS160E/FS160E.pdf
TYVM for the info & resource!