Previously, I made the case for why it is important to do a full repot (not to be confused with potting-up) and prune the roots of your containerized trees - regularly. Root-pruning is the systematic removal of the largest roots in the container with emphasis on removal of rootage growing directly under the trunk and at the perimeter of the root mass. The following is written primarily to offer some direction in the root-pruning of a high % of deciduous material, but with some very minor adaptations, it can be applied to conifers and evergreens.
Root pruning can start immediately with year-old seedlings by removing the taproot just below the basal flare of dormant material, repotting, and treating the plant as a cutting. This will produce a plant with flat rootage that radiates outward from the base and that will be easy to care for in the future.
Young trees (under 10 yrs old) are nearly all dynamic mass and will tolerate root-pruning well. Most deciduous trees are extremely tolerant of root work. Acer buergerianum (trident maple) is routinely reduced to a main trunk with roots pruned all the way back to the basal flare and responds to the treatment with a fresh growth of fine, fibrous roots and a fresh flush of foliage each spring. The point here is, you don't need to be concerned about the pruning if you follow a few simple guidelines.
First, undertake the root-pruning and repot while the plant is quiescent (this is the period after the tree has met its chill requirement and has been released from dormancy, but has not begun to grow yet because of low soil temps). The ideal time is immediately before buds move (swell) in spring - next best time is at the onset of budswell - next best time is anytime late in the quiescent period.
For deciduous plants that have not been root-pruned before: With a pruning saw, saw off the bottom 1/3 to 1/2 of the root ball. With a hand-rake (like you use for scratching in the garden soil) or a wooden chopstick, remove all the loose soil. Using a jet of water from the hose and the chopstick, remove the remaining soil - ALL of it. This should be done out of sun and wind to prevent the fine roots from drying. 5 minutes in the sun or wind can kill fine roots & set the tree back a week or more, so keep roots moist as you work. After the soil is removed, remove about 1/2 of the remaining mass of roots with a sharp pruning tool, taking the largest roots and those roots growing directly under the trunk. Stop your pruning cuts just beyond where a smaller root branches toward the outside, off the root you are pruning. Be sure to remove any J-roots, encircling roots, or others with abnormal growth.
Before you begin the pruning operation, be sure you have the soil & new container ready to go (drain screens in place, etc). The tree should fit loosely inside the walls of the container. Fill the container with soil to the desired ht, mounded in the center, & place tree on the mound. Add soil to cover roots & with the chopstick, work soil into all voids in the roots, eliminating the air pockets and adding soil to the bottom of the basal root-flare. Temporarily securing the tree to the container with twine or small rope, even staking, against movement from wind or being jostled will speed recovery time by preventing breakage of newly forming fine rootage. Place the tree in shade & out of wind until it leafs out and re-establishes in the container.
The first time you root-prune a tree will be the most difficult & will likely take up to an hour from start to finish, unless the tree is in larger than a 5 gallon container. When you're satisfied with the work, repot into a soil that you are certain will retain its structure until the next root-pruning/repot. Tree (genetic) vigor will dictate the length of time between repots. The slow growing, less vigorous species will likely go 5 years between repots. For these slow growing trees, it is extremely important that soils retain aeration. For these trees, a soil of 2/3 inorganic parts and 1/3 organic (I prefer pine or fir bark) is a good choice. The more vigorous plants that will only go 2 years between repots can be planted in a soil with a higher organic component if you wish, but would still benefit from the 2/3 inorganic mix.
Most trees treated this way will fully recover within about 4 weeks after the spring flush. By the end of 8 weeks, they will normally have caught & passed in both development and in vitality, a similar root-bound plant that was allowed to remain in its old soil and container.
When root-pruning a quiescent plant, you needn't worry much about "balancing" top growth with rootage removed. The plant will tend to only "activate" the buds it can supply with water. It is, however, the optimum time to undertake any pruning you may wish to attend to.
This is how I treat all my deciduous material. Though I have many growing in bonsai pots, more of my plants are in nursery containers or terra-cotta and look very much like your trees, as they await the beginning of training. With a little effort at developing a soil from what's available to you and some knowledge and application of root-pruning and repotting techniques, I'm absolutely sure that a good % of those nurturing trees in containers could look forward to results they can be very pleased with. This is the repotting technique described that allows bonsai trees to live for hundreds of years & be passed from generation to generation while other containerized trees that have not had their roots tended to, and have only been potted-up, are likely to be in severe decline or compost well before they're old enough to vote. ;o)
I hope you're bold enough to make it a part of your containerized tree maintenance, and I hope what I've written makes sense - it's well past a prudent bedtime for me.
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