To pot or not

Why grow nursery trees in containers? By Anna Mouton.

The traditional way to produce a nursery tree is to grow it in a field and lift it during dormancy. The biggest problem with this approach is transplant shock. Root damage during lifting is inescapable, and often costs the young tree a substantial part of its root system. This disturbs the balance between the root system and the top growth — the tree has to regenerate its roots before it can put on significant canopy growth.

“I’ve visited many orchards at the end of the season where there’s less than ten centimetres of terminal extension growth after transplanting nursery material. And that’s purely a function of transplant shock,” says Prof. Todd Einhorn of the Department of Horticulture at Michigan State University.

Einhorn believes that container-grown trees offer a viable alternative to bare-rooted trees. He shared the results of trials comparing trees grown in Ellepot containers to bare-rooted trees.

Putting pots to the test

The Ellepot system consists of a plug of growing medium held within a biodegradable liner and supported by an outer plastic container. There are several air gaps between the liner and the outer container. Root tips that grow through the liner die on exposure to the air gaps. The result of this so-called air pruning is increased new root formation.

Einhorn experimented with nursery trees of three apple cultivars grafted on the M9-clone Nic 29. Nursery trees from Ellepots all had significantly more fine roots and a greater total root length than field-grown trees of the same age. Honeycrisp and Fuji grown in containers also grew larger canopies. Canopy development in Gala was similar in Ellepots and field-grown trees.

The nursery trees were subsequently planted out at three different orchard sites. The trees from Ellepots had grown up to 150% more than bare-rooted trees after the first year. Ellepot trees had more shoot and leader growth as well as more branches per tree. Trunk diameter was correspondingly greater in Ellepot trees. They also had more fine roots.

“This year is the third leaf,” says Einhorn. “We’ll be harvesting fruit. Next year I expect the first significant crop.” He plans to compare yield data for three cropping years. “We want to show whether the early growth benefit of the Ellepot results in earlier yields and precocity, and pencils out.”

Einhorn cautions that more research needs to be done. The size of the nursery tree produced in an Ellepot is limited by container size whereas the same is not true for field-grown trees. “Do you lose by planting a smaller plant versus planting a bigger tree bare-rooted that goes through transplant shock? It’s an untested area in a side-by-side comparison.”

The pros of pots

Preventing transplant shock is not the only advantage of using containers. Prof. Karen Theron is the Hortgro Chair in Applied Preharvest Deciduous Fruit Research at Stellenbosch University. She remarks that potted trees can be planted out at any time of year.

“I would like to see growers planting before the winter, in April, so the tree establishes and develops a decent root system through the winter, and then takes off in spring. But you could also plant at any other time, for example to replace a tree that’s died.”

Theron also thinks that containers could be valuable for rootstock production. “One reason to go to containers is if you want to make clonal rootstocks using tissue culture.”

Better tree health is another potential benefit, says Theron. “You can use pest- and pathogen-free medium in pots, whereas the same fields are often used to produce more than one group of bare-rooted trees. The fields are fumigated but problems with apple replant disease can still occur.”

Theron has long advocated for growing nursery trees in containers. She argues that this is an established practice in the citrus and avocado industries. Some South African nurserymen have begun to produce deciduous fruit trees in bags with great success.

So why hasn’t everyone converted to containers? As with most things, there is some fine print.

The snags of bags

Cost is a frequently-raised objection to producing trees in pots as trees in containers may have additional production and transport costs. However Theron and Einhorn agree that the increased cost of container-grown trees is modest and more than offset by improved tree performance.

This performance depends on correct nursery practices. Trees that become pot-bound or develop circling roots establish poorly and will not reach their full potential. Ellepots and other root-pruning and root-training systems are designed to mitigate this risk.

Containers also pose other challenges. The small volume of growth medium increases the risk of drought and nutrient stress if irrigation and fertigation are not carefully managed — but there is also an opportunity for boosting growth through optimal watering and feeding.

Potted trees are frequently kept under shade net. They grow rapidly but can be weak and prone to falling over if not supported. They may need protection from full sunlight if they are planted out while still in growth.

Einhorn emphasises the importance of irrigation when planting out potted trees. “The porosity and density of the media are very different from the native soil. Despite compacting at the interface when planting, you can get drying between the different soil types and it limits conductance of water.”

Einhorn recommends irrigation using drippers positioned directly over the root system if possible, and making sure that the watered area extends beyond the root ball. This will encourage the tree to root into the native soil.

Both Theron and Einhorn acknowledge that there are still many unanswered questions around best practices for producing nursery trees in pots.

“Our future work will attempt to show what are the weak links in the system. And then my interest is to try to push multiple growth cycles in a season — to get a two-year tree in one year,” says Einhorn. “I’d like to try and get a bigger tree that has more fruiting wood and could potentially give earlier yields. But that’s all for the future.”

Meanwhile pots are becoming more common on the South African scene and look set to play an important role in the success of our deciduous fruit industry.

Image: Container-grown trees at a South African nursery.

Supplied by Karen Theron | Stellenbosch University.