The Best Time to Plant
We have routinely advised growers in the upper Midwest to plant bare rooted trees mid- to late March, April or June (June? Why not May?). Several research studies have demonstrated the advantages of planting as soon in the spring as the soil conditions will allow. Trees planted in April have a decided advantage over those planted even one month later. As temperatures increase in late spring, trees planted late will break bud sooner and struggle initially without a regenerated new root system developed. Avoid planting trees in frozen or water-saturated soils. Some growers have experimented with fall planting, but this method has its risks associated with subjecting young trees to severe winter temperatures. Additionally, many nurseries cannot sell and ship these trees in time for a fall planting. We have tried planting trees in mid-November and were pleasantly surprised with the outcome of some Tall Spindle apple trees. Trees that were not pruned following planting not only survived fully, but had a crop of several fruit in the first growing season.
Upon receipt of trees in late winter/early spring, inspect them for root health and moisture. Sometimes trees shipped begin to dry in transit or in temporary storage venues. Unpack the trees from their containers and add moisture, moist shavings or shredded paper to insure roots are not dry. Hold them in humid cold storage above freezing temperatures until you are ready to plant. Do not store trees in fruit cold store rooms where ethylene from the fruit remains in the storage unless the room is well aerated. The slightest exposure to ethylene can stimulate bud break. Always keep trees moist and protected from freezing temperatures. Trees that are kept for more than a few days should be “healed in” in a well-drained location using moist light soil, wood shavings, sand or sawdust. Inspect and count trees immediately after delivery. Call the nursery if you discover a problem with your trees upon arrival. Trees should not be stored in boxes longer than five days without adding additional moisture around the roots. Make sure trees are hydrated within a day or two prior to planting using oxygenated (flowing) water for four to 20 hours. Check tree roots for crown gall disease caused by Agrobacterium tumefaciens which causes disease in many fruit and nut tree species (dicotyledonous). Report the incident to the nursery that provided the trees. Infected trees have tumor-like swellings called galls on the crown of the plant just above the soil. Crown gall disease does not usually seriously harm older plants. Tree roots can be dipped in Agrobacterium radiobacter isolate K-1026 (commercial product known as NOGALLTM (often accomplished at the nursery) to suppress crown gall.
Preparing the Planting Site
Many gardeners recommend the use of compost in the planting hole to improve root regeneration and initial growth. Make sure that the compost is aged and avoid green vegetation when working soil. Do not fertilize trees at planting time. Instead, if a grower needs and wants healthy growth, wait until May or June when trees have begun to grow. Use a balance fertilizer with light amounts to avoid burning roots (particularly a problem in sandy soils).
Digging the Hole and Planting the Tree
Your soil should be dry enough to easily crumble. The holes should be dug just prior to planting to minimize sidewall glazing. Glazing can also occur when an auger is used but can be avoided with auger bits which have a scarifier welded to the edge of the bit. Glazing can also be avoided by scarifying with a shovel. Dig the hole slightly larger and deeper to accommodate the roots. Holes that are too deep may cause trees to settle too low after irrigation or rainfall. Broken or damaged roots should be trimmed off. No additional pruning is recommended. Do not prune the roots to fit the hole; instead, dig the hole to fit the roots. Remember to remove tree labels to avoid trunk girdling. Backfill the hole half-way with the most friable soil available and avoid large clods. Tamp soil around backfill soil to extinguish air-pockets around roots. Backfill the remainder of the hole and repeat the tamping down step. Watering immediately afterward effectively completes this process. The tree depth should be adjusted so that after the soil is settled the bud union between the scion and the rootstock is approximately 3 to 4 inches above the final soil line. Setting the bud union higher than this can lead to more dwarfing of the tree. An old rule of thumb was that for every inch the bud union is above the 3-4 inch height will reduce overall tree size by 10%. Some commercial growers will purposely put the bud union higher to obtain additional size control However, exposure of additional rootstock stem tissue can lead to the formation of burr knots and potential sites for dogwood borer, Synanthedon scitula (Harris), infestation.
Trees can be leaned at planting time towards the west or southwest on windy sites. Planting trees at an incorrect depth can be a serious mistake that can ruin a new orchard for life. Generally, this mistake is most serious if trees are planted too deep. Shallow planted trees can usually be corrected and have less serious consequences, if there are any at all. The consequences for deep planted trees can range from them having a slow start to scion rooting, particularly to apples. Deeper portions of the soil profile possess more free water, lower temperatures and less oxygen. These conditions inhibit new root growth and lend to a slower start for trees. A grower may not intentionally plant trees excessively deep. Trees that are augured in (trees planted in holes made with an auger) have a tendency to settle following rains or irrigation as much as three to five inches in depth, depending on soil type. Mechanically planted trees will settle less at two to four inches.
Problems: Scion Rooting
Scion rooting of apple trees on dwarfing rootstocks is a serious and common problem caused by deep planting. At the time of planting, the orchardist may have thought that the trees were planted correctly, only to find out five to seven years later that the unions are not visible and trees appear abnormally vigorous. As a consequence, adventitious roots arising from the scion become dominant and the trees take on the normal vigorous level, which approximates seedling. Unfortunately, at that point, nothing can be done to correct the problem. Digging down to expose the unions and cutting roots causes a depression in the soil which allows water to fill causing Phytophthora crown rot or later in the year, ice formation and subsequent injury to the crown. Secondly, the depression refills with eroded soil later in the year anyway, requiring follow up attention.
A standard recommendation for apples has been to plant trees that are on clonal rootstocks so that the union is a minimum of three to four inches above soil line. As a tip and if you are supervising a crew of workers, have them use a 2”X4” (two to four foot long) piece of wood to help as a guide. Place the 2”X4” board on its edge adjacent to the planted tree (perpendicular to the row) to check its depth. The union should clearly appear above the edge of the board indicating that it is at least four inches high. Experience thus far with apples is that trees are most vulnerable between years three and seven to eight. Before year three, burr knot development is minimal in most cases and after year eight primordial root initials in the knot area are hardened and appear less attractive to dogwood borer larvae. However, the trees are just as vulnerable to scion rooting, which can be a more difficult problem to address and correct.
A shallow planted tree can be easily corrected later in tree life, compared to the task of trying to correct an insurmountable problem of a deep planted tree. Using a mechanical planter, growers should be advised that someone should follow up and check each newly planted tree to insure proper planting depth. While the soil is still loose, pull trees up that are too deep and push those down that are too shallow. When setting planting depth for mechanical planters, stay on the shallow side. If you error, do it on the side of shallow planting. The last thing you need five years down the road is to have trees planted five feet apart in a high density planting that should have a spacing of 15 by 20 feet!
In rootstock trials, the rootstocks that produce the largest area covered by burr knots are Mark and M.26. The rootstocks M.26 and M.9, which also has a tendency to produce burr knots at a high rate, depending on the clone, make up more than 50 percent of new trees being planted today in North America. Therefore, for newly established orchards it is sometimes recommend that trees be mounded (bermed) within one year following planting to avoid dogwood borers. Mounding can also help trees avoid Phytophthora infection in finer textured soils by encouraging water to drain away from the trunk/ground interface. Some growers have started mounding soil on tree trunks where trees are weak as in apple on Mark or B.9 rootstock or in some cases, where trees are planted on M.9 with a weak growing scion or in a droughty soil.
Generally, it takes a few years before the adventitious roots from the scion begin to influence scion vigor. Unfortunately, this strategy can yield unpredictable results, depending on the rate and intensity of root development and on the soil conditions. Plum growers have had some success in mounding soil on the trunk of “Stanley” and encouraging scion rooting. Eventually scion roots take over and provide a bypass around occluded unions in cases of brown line disease caused by tomato ringspot virus.
Care of Newly Planted Trees
The most important after care for young trees with limited and shallow root systems is to keep moisture levels as high as possible without excessive wetness. Once planted, if a trickle system is not ready to apply water, tank water individual trees. Basins may be necessary to hold water near the backfilled area. A light irrigation or tanking with three to five gallons of water is recommended. This will eliminate the air pockets around the roots. Subsequent irrigation should not be applied until after new growth has started; trees can be killed when too much water is applied. If the trees have settled after the first introduction of water, they should be pulled up immediately, rather than after they have started to grow. Then, re-tank with minimal water to settle soil around so that the hole walls are glazed over by mechanical auger bits.
Jon Clements, University of Massachusetts Amherst
Dr. Michael Parker, North Carolina State University