When a healthy plant has been growing for years in one spot, its above-ground and below-ground structures and systems are in balance. Digging, lifting and relocating cause root loss which diminishes water intake capacity. Equilibrium is disrupted.
The extent of root damage, its impact and speed of recovery are related to plant size, whether the transplant has a woody or soft stem structure and plant physiology. Food-making takes place in plant leaves. During photosynthesis water and oxygen molecules not used to make food are released into the air creating a constant demand on roots for moisture. Within limits, intense sunlight and high temperatures increase the speed of photosynthesis, drawing more water to the foliage. Dry winds also deplete moisture in leaves and stems increasing the soil water intake demand.
During spring and summer, plant energy fuels shoot, leaf, flower, fruit, seed and root growth. Carbohydrates are stored, too. After leaves drop in autumn, the pressure on roots of deciduous trees and shrubs to bring in water lessens. More energy is available for root growth. Needle and broadleaf evergreens have leaves year-round, but because the sun is not directly overhead and air temperatures are lower in wintertime, the roots are less involved in food manufacture and have a growth period, as well.
Overcrowded, herbaceous perennials like iris, daylilies and tall garden phlox can be dug for transplant in summer, with careful hot weather management. They bounce back quickly unlike trees and shrubs that can suffer extreme heat and drought stress. When a woody-stemmed transplant candidate is young and small, chances of a successful move are better than those of bigger ones. The extensive top-growth of a larger, mature specimen depends on a correspondingly large root mass to keep the whole plant operating efficiently.
Use warmer months for preparation (researching plant sun and moisture requirements, picking the best available spot, root pruning and digging the destination planting hole) but wait for cooler temperatures after plants go dormant to relocate sizable transplants. The ground rarely freezes during winters in Tennessee. That provides an extended period for root growth. Trees and shrubs need about 2 years to get re-established.
Small plants typically have a root mass which extends just beyond branch tips (a point referred to as the "drip line") in a deep, rounded formation that mirrors the shape of the crown. Woody plants were traditionally thought to be similar in form, but modern research suggests tree roots have a profile that is more pancake shaped, extending out many feet on all sides of the trunk but not very far down.
The fact sheet "Transplanting Trees and Shrubs" by Cornell University Extension (online at http://rocklandcce.org/PDFs/Horticulture_Fact_Sheet_205.pdf) says lateral roots on a 12-foot tree may extend 9 or more feet out from the trunk. Tiny, feeder roots whose purpose is moisture intake grow at the tips of older roots. They occur a few feet out from the trunk within 18 inches of the soil surface. Water-absorbing roots for shrubs are found in the upper 8 inches of soil.
Attempts to preserve all tree roots with the soil attached would result in a root ball so awkwardly wide and heavy it could not be lifted or transported. Guidelines specify the diameter to be left behind in a move. Keep transplants consistently well watered while their root volume is being replenished and root efficiency is impaired.
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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) for answers to horticulture questions, free publications and how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, MGardenerRae@frontiernet.net.