Transplant Success of Field Grown Trees

Issue: 
March-April 2006

HAVE YOU EVER WONDERED if there was something you could do to improve transplant success of your new tree installations each year? There is. Actually, there are lots of ways to help ensure that newly-planted trees have the greatest chance for survival. A better understanding of root and shoot growth patterns, nursery production practices and cultural management techniques can go a long way in reducing the number of failed installations.

It becomes painfully clear each year that some species of trees are easier to transplant than others. In addition to rapid root regeneration, the amount of fibrous roots within the natural root zone may help explain why. Think about the fibrous root systems of Cornus, Fraxinus and Tilia and how easy these plants establish. There are exceptions however, Betula have fibrous root systems yet they don’t respond well to transplanting. Research at the University of Guelph rhizotron indicated that Betula are very slow to begin root growth in the spring. It is the fibrous roots (also called feeder roots or root fans) that are critical for the uptake of water and dissolved nutrients (Photo root fan). Fibrous roots are found mainly in the top 30 cm of the soil. This is especially true if the soil is shaded, mulched or has small plants growing at the surface. (It is interesting to note that root densities are highest on the north side of the tree.) The fibrous roots are found near the surface of the soil because this surface soil is more apt to meet their needs for water, nutrients, oxygen and heat. Roots will grow out from the trunk in response to these items. Roots of large, established trees will often extend outwards, two to three times the radius of the crown, in order to meet their requirements. So if you want to water the feeder roots of an established landscape tree, direct your irrigation within and well beyond the drip line of the canopy. The same goes for fertilizer applications.

When is the best time to plant to take advantage of root growth periods?
It is difficult to give a solid threshold to mark the beginning of root growth in the spring. Based on scientific literature, it seems that root growth in most deciduous species is well under way when soil temperatures (at a depth of 20 cm) exceed 10oC. My guess is that we reach this point around mid to late May in southern Ontario. Many conifers exhibit root growth and activity at even lower soil temperatures. Shoot growth (bud swell, bud scale splitting, leaf emergence) can begin before root growth, and can precede or coincide with the first flush of shoots on deciduous trees. Trees that are transplanted in early spring must initially rely on older roots for water uptake to support the shoot emergence. Once the leaves flush out and soil temperatures warm up, a period of heavy root growth begins. Root growth is driven by the sugars manufactured in the first (and second) flush of shoot growth. Trees that are transplanted in the spring have a greater chance at establishment because of the root growth periods throughout the first growing season. In the autumn, root growth gears up again as shoot growth ceases and foliage begins to drop. This is why soil moisture is so important in the autumn months, especially in the year of transplanting.

In autumn, soil temperatures below 10°C will stimulate root growth to shut down on most deciduous trees. So from a physiological point of view, fall transplanting of trees as the foliage is starting to turn colour (e.g. end of September) should give the greatest opportunity for root growth before plant dormancy. However, attention must be paid to minimize water loss from leaves and exposed roots. This is where anti-desiccants and mechanical leaf removal can play a role. Planting in early October should still provide enough time for root regeneration before freeze up. Trees that are transplanted very late in the autumn seem to fare poorly as they must rely on the portion of older roots (which were retained after harvest) to support them until root growth resumes the following spring. Check your favourite horticultural book or website for a list of trees that are suitable for fall planting.

Have the trees been root pruned at the nursery?
Where roots are not pruned prior to harvest, most of the roots may be left behind after digging the ball. Several references state that only 3.8 – 8.5% of the root system actually accompanies a dug caliper tree to its planting site. As long as soil moisture is not limiting, most trees are able to flush out and grow to replenish their root systems. However, trees that are transplanted with little of their original root system are at a disadvantage and establish poorly in sites where soil moisture is limiting (think back to 2005). Without a successful first flush, those transplants will have a reduction in photosynthate that is required to support subsequent root growth and the downward spiral continues. Practices that can conserve the number of these fibrous roots on a harvested specimen will result in a plant that establishes faster after transplanting. In the nursery, root pruning or trenching along the root system just inside the area where the root ball will be dug can increase the density of fibrous roots in the harvested portion of the root system. This, in turn, can increase the chances of transplant success.

Should we hand prune roots and shoots at the time of planting?
Bare root trees are commonly hand pruned before storage and planting. The purpose is to make a clean cut where roots are damaged and to ease storage and planting. Since a reduction in roots prior to planting can reduce the supply of water and nutrients to the canopy, we have to wonder what kind of effect we are having on the transplant success of these trees in low/no maintenance areas.

Some horticulturalists believe that by pruning some of the canopy away at the time of planting, that there would be less demand on this smaller root system. However, science tells us that by removing young shoots of deciduous trees, we may also be removing the source of auxin (in buds, leaves) that would normally stimulate root development. In addition, canopy pruning will result in a reduction of photosynthetic activity (food supply). Although it is wise to prune out damaged branches and remove branches that will interfere with architecture and long term health, it is advisable to keep canopy pruning to a minimum in the first year after planting.

Is Post-Planting Irrigation Necessary? What about fertilizer in the first year?
Adequate soil moisture is undoubtedly one of the most important factors in transplant establishment. Absorption of water from the soil occurs primarily through fibrous, non-woody roots. As previously discussed, harvested field grown trees have had a severe reduction in the number of fibrous roots. Regular, deep watering of newly planted trees will definitely help decrease water stress and aid root regeneration (where soil moisture is limiting). Our traditionally cool, wet springs are usually quite conducive for root regeneration. However, we did experience some unusually dry conditions in the spring of 2005. Most of the literature agrees that fertilizer applications in the year of transplanting are not nearly as important as supplemental irrigation.

It’s Spring! Time for Dormant Oil.
Dormant oil is an effective, low toxic way to minimize overwintering popula-tions of immature insects and mite eggs. I’ve heard great things about dormant oil helping to manage magnolia scale, euonymus scale, spruce spider mite and spruce gall adelgid. Remember, insects like to hide on the undersides of twigs
and foliage. Direct the application of horticultural oils to maximize contact with insects and mites. As for delayed-dormant and summer applications of horticultural oil, watch for Landscape Oil to hit the market this year. For more information, stay tuned to the Nursery-Landscape Agriphone message.

Dormant oils provide a physical barrier which restricts both the respiration and movement of many overwintering insects (e.g. adelgids, scales, mites). To avoid foliar damage, dormant oil is best used when min/max temperatures remain somewhere between 5 and 15oC during the first week or so after application. This is especially important on evergreens since their foliage is always present. Try to make applications when conditions will facilitate rapid drying of treated twigs. This will also help minimize phytotoxicity problems. Avoid mixing oil with sulphur and other fungicides as plant damage may result.

Always read the label carefully since many plant species are sensitive to dormant oils (see below). I’ve had professionals tell me that they have used dormant oil Taxus and Thuja for years and never had a problem. Others have seen quite a bit of burn on those two genera – yikes. Remember, oil and water do not mix. You will need to agitate the mixture constantly in order to ensure even coverage. If over application occurs and extreme temperatures are experienced within a week or two, foliar damage may result. Applications should be made during mild mornings when no rain is in the forecast, to facilitate drying.

Plants Sensitive to Dormant Oil Treatments
Acer palmatum (Japanese maple), Acer rubrum (red maple), Acer saccharum (sugar maple), Carya (hickory), Cryptomeria, Juglans sp. (walnut), Juniperus (blue cultivar selections), Picea pungens glauca (blue Colerado spruce), Pinus strobus (white pine), Quercus rubra (red oak), Taxus (yew) and Thuja (cedar).

Other, Less-Sensitive Plants to Dormant Oil Treatments
Cercis canadensis (redbud), Fagus (beech), Ilex crenata (Japanese maple), Picea abies (Norway spruce), Picea glauca (white spruce), Pseudotsuga menziesii (Douglas fir).


— Jen Llewellyn, Ontario Ministry of Agriculture and Food (OMAF) Nursery Crops Specialist

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