The two traditional methods of tree and shrub production - field growing and container growing - each bring their own advantages and disadvantages to nursery production. The former offers a short sales and planting window, for example, but while the latter extends that period, plants grown in containers can suffer from pot-bound roots and will often blow over in heavy winds.
There is, however, an alternative that claims to combine the advantages of both systems while reducing the disadvantages to produce a superior product. The pot-in-pot system has become popular with US growers since its introduction in the 1980s, and UK nurseries newly turning to the method are already reporting greater efficiencies, better-quality plants and cost savings.
The pot-in-pot system uses two containers - a socket pot that stays permanently in the ground and a smaller inner pot, which sits inside the socket pot when planted.
The system is commonly used in the US for larger trees and shrubs and, while sizes can be altered to suit smaller plants, cost is often prohibitive. The common pot sizes used are 35 litres for the socket and 25 litres for the inner pot to produce quality, well-branched specimen shrubs of around 2m in height.
The method of production offers several advantages over traditional container production. The rootball of the plant is more protected from extremes of temperature in both summer and winter, and irrigation costs can be reduced with the adoption of trickle systems. The need for staking is removed as blow-over is rare, and labour costs are reduced. Possible damage to the rootball associated with field-grown harvesting is also eliminated.
Many US universities including Tennessee, Kentucky and Georgia, as well as the US Department of Agriculture, have researched the method, examining economics, cultural practices and plant quality. The findings show that although initial set-up costs far exceed field production and normal container production costs, the results of moving over to the system are superior plants that grow quicker, require less labour and fetch a premium price.
Kent-based Palmstead Nurseries started selling volume stocks produced this way in 2008. Joint managing director Bob Chapman says implementing the system has allowed for a very economical layout of plants over a large area while giving plants enough space to fill out into a quality product.
"As a result, our customers end up with a much better product and we can get a better return for it," he says.
The nursery uses the system to produce 2,000 plants on 0.3ha, and aims to increase that area in future. To make the system as profitable as possible, the nursery uses it for volume lines such as Elaeagnus, Photinia, Prunus lusitanica Award of Garden Merit and Amalanchier.
Drainage and irrigation are the primary cost factors in setting up the system. For a nursery like Palmstead, working on free-draining sandy soils, this will be less of a factor. But on heavy soils drainage lines ideally need to be placed under every row of containers. Research from the US shows that even a slight slope in the production area can cause water to sit in the bottom of the pots, so it is best on levelled ground.
Blow-over is virtually eliminated but can still happen in extreme winds. To prevent this, Palmstead ties each row of plants to a horizontal wire - an easier, efficient alternative to staking each plant individually.
Trees and shrubs raised in pot-in-pot production also tend to grow faster, often reaching saleable size in half the time required with field production. The grower can also add fertiliser and water to the exact requirements of the plant through trickle systems.
Early growing in the US showed that even with irrigation, plants were prone to sending out roots, commonly through the first pot, but also through the socket pot into the ground. Palmstead has combated this by placing a barrier in between the two pots to prevent anchoring roots growing out into the ground. By using material from a grow-bag system specifically developed to restrict root development, only small aerial roots are able to escape into the socket pot.
"The addition of a barrier between the pots prevents larger roots from growing out of the container," says Chapman.
"We use a square of Caledonian Tree Company's Superoots Fielder root-control bag - fine roots get through but the material's constrictive nature prevents larger anchoring ones from emerging. Allowing the fibrous roots to penetrate means there is no ringing of roots inside the pots and the plants transplant much better as a result."
Roots that make it through the fabric are restricted to a 3mm diameter as the weave of the fabric will not stretch any more than that. The 3mm restriction means transplant shock is minimal and the nodules formed behind the restriction contain high levels of carbohydrate or starch, resulting in a more rapid and successful establishment rate.
The few escaping roots become limited in length but they continue to source water and nutrients. The system greatly reduces planting stress, which Chapman says means low maintenance and replacement costs - and happy clients.
He adds that it is not economical to transport small orders from the grow site to the dispatch area. Instead, plants are collected at a rate of 50 to 100 specimens per species at a time and held in the sales area closer to dispatch.
Production time depends on the variety being grown but generally most plants are planted up from a three-litre container and grown on for one year to 18 months.
Chapman says: "We time production so the plants reach their final sale height after the first flush of spring growth. Once they are through this we select the best 100 of each genus and move these to a prominent site on the nursery. By the time they are sold, the other plants have filled out and we select the next 100 to be moved to the sales area and so on."
It is possible to produce a good 1.5m laurel in a 10-litre pot, but Chapman says there is growing demand for UK-grown 25-litre specimens more commonly supplied by Continental producers, and that the pot-in-pot system allows the nursery to produce a taller 2m plant in a 25-litre container for very little extra work.
"There is increasing demand for larger container plants and we expect that to continue," he says. "Continental stocks are looking less viable as we see landscapers looking closely at costs. The current euro exchange rate is making Dutch stocks up to 25 per cent more expensive and landscapers are increasingly turning to UK produce. The Dutch are good at responding to market conditions with price adjustments, but it becomes less viable once they have to start cutting prices by more than 10 per cent."
Future profits for Palmstead pot-in-pot plants look good despite uncertainties in the current economic climate. "The system brings better returns than a field-grown tree crop on the same scale and there is also less labour involved," Chapman says.
"It generates a better return per square metre compared with a traditional Mypex-covered container bed system - and it's cheaper to set up, too. It's generally a cheaper way of growing a quality plant."
BENEFITS OF THE POT-IN-POT SYSTEM
- Insulates the rootzone from extreme temperature variations
- Allows in-place over-wintering
- Decreases production time from liner to finished product
- Reduces water usage with trickle irrigation
- Eliminates the need for staking to prevent blow-over
- Reduces labour costs associated with in-field harvesting
- Prevents root loss associated with in-field harvesting