Friday, April 28, 2006
An Arbor Day primer: making roots happy
Since today is Arbor Day, a day dedicated to planting, nurturing and celebrating trees, let's discuss the essentials of tree root health.
Every tree depends on a healthy root system. As with most plants the real action with trees is down in the soil. While it's easier to focus on the trunk, crown and leaves, if the roots are happy chances are the whole tree will thrive.
The first thing to understand about a tree root system is that it starts right at the surface of the soil. This makes sense when you consider that a maple or white pine seed will simply flutter to the ground and if lucky will land on some bare ground and get started. An acorn buried by a forgetful squirrel has the energy stores to push up through a couple of inches of soil, but this is still starting pretty much at the surface.
The average nursery tree
This contrasts sharply with the average nursery tree. In the nursery trees are replanted frequently to space them out as they grow — each time a little bit deeper. Machine cultivation for weed control and "root pruning" often throw loose soil on top of the original soil surface. When the tree is dug and packaged, the burlap wrapping tends to bunch up soil from the outer edge onto the top surface again, which in effect lowers the tree further yet within the ball.
By the time a nursery-grown tree is delivered to the landscape, its root system might already be buried by several inches of soil. If the tree is planted too deeply, its main root system will start to decay for lack of oxygen. Secondary roots are typically able to rise to the surface for sufficient water and nutrients, but the tree must spend precious energy reserves battling the new decay. This root decay also makes an otherwise healthy looking tree vulnerable to blow over in a storm. Recognizing this persistent problem, the nursery industry has adopted strict standards for root ball configuration and true depth relative to tree size and shape, but actual change has been slow.
Properly elevated trees
Compounding these unfortunate nursery practices is the tendency of landscapers to plant trees too deeply. Arborists rant and rave about this very common problem at every opportunity in the hope of educating the general landscaper. I've come to the conclusion that the practice is often deliberate. The truth is that a properly elevated tree will require more frequent watering and possibly staking. If quick installation and warranty survival of one or two years are the real goals, then sinking trees makes perfect sense.
Even with the best intentions, however, it is easy to sink a tree too deeply. Once the dirt starts flying it's hard to keep your vertical orientation. The trick is to use a couple of stakes, some string and a tape measure. Measure the height of the string to grade level before digging, add the depth of the pot or tree ball and subtract for all that extra soil that might be on top of the root system. This should be the distance between the string and the bottom of the hole. Dig, check and repeat. You will be surprised how shallow this measured hole will be. Spend your extra energy digging wide for root expansion. A shallow dish-shaped hole will also prevent the tree from sinking when the soil settles, which is another common problem.
Trees need their own space
The next thing to understand about tree roots is that they generally stay in the top 1/2 foot to 1- 1/2 feet of soil. Just as you would not plant tomatoes in the middle of your lawn and expect them to survive, trees also need their own space. The advantage of a tree in a forest is its ability to conquer grass by shading it into oblivion. We subvert this strategy, however, with our suburban landscaping, and the roots of many trees that we care most about are in direct competition with turf for both water and nutrients. Plenty of well-adapted trees will cope without complaint, but many others will become vulnerable to insects and fungal decay precisely because they need more root space. Love that tree? Kill the grass.
Mulch circles are great, the bigger the better, but I'll admit they look too big once they are past the crown's "dripline." Just make sure not to overdo it with the thickness two to three inches are ideal.Pictures of "mulch volcanoes" are guaranteed laughs at any lecture on plant maintenance. If the mulch needs to be refreshed for cosmetic purposes, the old mulch, typically shredded bark, should first be removed. It can usually be put to great use someplace nearby that is less aesthetically demanding. A lot is written about mulch layers retaining moisture and keeping the soil cool but the most important function is simply eliminating competition from weeds and turf. A thin layer of dead leaves, compost and/or woodchips combined with an occasional attack on emerging weeds with a hoe or other handy weapon are just as effective as a thick blanket of shredded bark.
Removing the burlap or pot
It is important to emphasize that roots don't like changes. The burlap surrounding the root ball must be removed from at least the upper half of the ball. This is frequently neglected in the belief that the burlap will decay over time; it will someday but meanwhile the roots will grow in a circle within the ball. To remove the burlap, string and possibly wire cage I first pack soil around the entire ball and then uncover one section at a time. It's very important to keep the ball intact throughout the entire operation. If the root ball is allowed to fall apart during planting or is mishandled in shipping, the crucial remaining root hairs are severed.
Many potted trees can be flipped and cradled to remove the pots and trim any circling roots, but sometimes it is best to plant the entire pot and then remove the vertical pot wall in sections while supporting the fragile root system. The pot bottom can be removed first or left in the ground.
If the soil within the dug hole is "enriched" again during planting, the roots will just circle around avoiding the poorer native soil. It is for this reason that the best method for fertilizing is typically working in a layer of compost and decaying mulch on the soil surface over the entire root zone.
The last crucial point concerns water. Most trees that die in the first two years are victims of some sort of water problem. A watering ring dam is probably the best method for irrigation. This temporary basin holds the water close to the tree as it slowly soaks into the soil. Be generous, but be careful not to drown the roots by too-frequent watering. This is hard to define, but if you actually dig in a few inches and feel the soil, you'll probably get it right.
The weather should be good now and the bugs not bad, so go out there and show your appreciation for the trees that grace our landscape. Find a tree that needs a new home or just a little home improvement, and have a great Arbor Day.
© 2006 The Carlisle Mosquito