Article by Bas van den Ende
Consultant in fruit production (ret.).
When you have planted fruit trees this winter, you may notice during the growing season, that some or all of the trees make poor growth. Very often, this problem is attributed to a phenomenon called transplant shock. Before you pick up the phone to call the nurseryman who grew your trees, you should know the facts about transplant shock.
Facts about transplant shock.
There are six possible reasons why newly-planted nursery trees can suffer from transplant shock:
- Pear and cherry trees are known to suffer more from transplant shock than plum, peach or apricot trees.
- Water movement from the soil to the atmosphere through a tree is a continuum. This continuum is broken when nursery trees are lifted and must be restored as quickly as possible after trees have been planted. For restoring the continuum, good root-soil contact is necessary; especially since the root surface area has been reduced as a result of lifting the trees from the nursery.
- Lack of oxygen in the soil and/or low soil temperature. Fruit trees have a high root oxygen requirement and roots function best at oxygen levels above 10%. It is possible that the water movement from soil to the atmosphere is established, but root growth fails. This failure of root growth may be caused by lack of oxygen due to waterlogged compacted soil or low soil temperature. Pear roots start to grow at 7-8°C, but the optimum temperature for root growth is 18 - 22°C. The quicker the soil warms up the quicker roots will start to grow. Cold and wet soil is not good for newly-planted trees.
- Too much top and not enough roots to balance the top. This problem is often the main reason for transplant shock. The roots store carbohydrates (starches) and nutrients. These storage materials are mobilised in the spring for growth of roots and shoots. As trees are liften in the nursery, a proportion of the roots is cut and remains in the soil. The bigger the top, the more roots should be retained. But this is not always the case, because the digging machine is often set at a certain depth. As a general rule, the bigger the trees are, the greater the chance of transplant shock. Small trees, or trees that have been cut back severely, seldom suffer from transplant shock.
- Trees have not been hardened off. While nursery trees grow, they do not store carbohydrates and nutrients in their roots and bark. It is only when the trees stop growing and are hardened off, that they build up their reserves, provided the trees still have many green leaves. Reserves of carbohydrates and nutrients are necessary for growth of the trees in spring. The trees must stop growing sometime in late summer so that they can start storing carbohydrates and nutrients. The trees have stored enough carbohydrates and nutrients when the leaves fall off naturally in early winter. If trees are allowed to grow well into the autumn and leaves are artificially removed with a spray or by hand, the trees do not have time to accumulate enough reserves for sufficient new growth next spring. Nurseries have some control over hardening off the trees by withholding late applications of nitrogen fertiliser and allowing the soil to dry out.
- Lack if chilling. Trees need a certain amount of chilling to develop normally in the spring. Chilling hours are accumulated in autumn and winter between 0 and 7°C. High-chill varieties for southern Australia need at least 800 hours of chilling. Less than 600 hours will cause the trees to develop very slowly in spring. This symptom is called “delayed foliation” or DF. Trees start to accumulate chilling hours when they stop growing and their leaves fall off naturally. Lack of chilling in combination with not enough carbohydrates and nutrients stored in the trees, can lead to severe transplant shock.
Transplant shock is not to be confused with:
- (a) Replant disease. When an old orchard is removed, some organisms and root toxins are left in the soil. When the same type of fruit tree is planted back into this soil, it can lead to poor growth. Apple trees are very prone to replant disease, unless the soil is first fumigated. Pears, peaches and plums do not seem to suffer a great deal from replant disease.
- (b) Nematode infestation. Nematodes are microscopic, worm-like animals that live in the soil. Most nematodes are harmless, feeding on decaying organic matter. But some attack the roots of fruit trees causing severe stunting. Nurserymen should not plant in nematode-infested soil. When replanting a nematode-infested block, orchardists should plant trees with nematode-resistant rootstocks.
How to manage transplant shock
It is not practical to determine the level of reserves in nursery trees and know whether those levels are enough for normal growth. As an orchardist, it is also difficult to know how many chilling hours the trees have had.
However, you can lessen transplant shock as follows:
- (a) Do not allow the roots to dry out after you have received them from the nursery.
- (b) Water the trees in when you plant the trees.
- (c) Reduce top growth when the trees are large and the roots are small. Do this by removing first the strong side branches (feathers) with bench cuts above knee height. Cut thin branches back to about 100 mm. Avoid heading the trees, unless you plant trees onTatura Trellis or grow the trees with several upright leaders.
- (d) Apply a restbreaker, such as 2 winter oils sprayed about 4 weeks apart, the last one just before the buds start to swell. Do not spray oil close to a dormant zinc spray.
- (e) If you suspect that the trees may not have had enough chilling, place them in cold storage in wet saw dust in bins at 4°C for 4 – 6 weeks. Do not store the trees in the same room with fruit.
- (f) After planting, keep the soil moist, but not wet.
- (g) Control weeds to eliminate any competition for moisture in the soil.
- (h) Do not put any fertiliser or animal manure in the planting holes.
The above measures are designed to prevent the newly-planted trees experiencing stress. Any stress can exacerbate transplant shock.
