By Fred Field, Horticultural Consultant
EverGrow Bags are designed to restrict root growth and as a consequence control the growth rate and fruitfulness of fruiting and ornamental plants.
The growth rate decreases as the size of the bag decreases.
Fruitfulness increases as the bag size decreases.
How does it work to restrict roots?
Basically, roots of a tree will not pass through a pore size (or hole in a bag) smaller than its own diameter. The root follows the path of least resistance then stops growing when it hits a pore size smaller than its current diameter. What is does next is to grow laterals. So roots of trees in bags are naturally contained by this process (which is in effect pruning of the roots). The root will not continue to circle the bag becoming root bound, but will naturally stop growing, thus affecting growth and fruit size above ground.
Nature plays a role in how roots perform. If it is very dry the plant certainly won't expend energy seeking something they instinctively know isn't there. In turn this will govern the activity above the ground. The obvious actions that take place when roots are restricted are usually as follows:
The bag has been designed to be used above ground or buried in the soil. Buried it should last indefinitely.
The density of holes in the bag slows drainage rate of water applied to allow even distribution of water within the soil in the bag without causing water logging problems.
When planted underground the diameter of the holes allows fine roots to grow through the sidewalls, however as they thicken they become 'choked' by the strong fabric. When used above ground, 'air pruning' ensures that no roots try to escape from the bag.
While the bag has been designed to provide satisfactory drainage rate it must be emphasized that the bag will drain no faster than the surrounding soil. Therefore, sites should not be used where normal soil drainage is poor, such as shallow, heavy clay soils or where perched water tables develop.
Where normal soil on the site is well drained and has characteristics for the species growing in it, the soil from the hole dug to bury the bag may be used as a root medium. However, a well-drained potting mix is preferable if there are any doubts about the suitability of such soil - and especially so, if the bag will be kept above ground. Potting mix should contain a good quality complete slow -release fertilizer mixed thoroughly into the medium. Soil medium which contains soluble clays, or clays which swell and shrink during wetting and drying cycles should not be used as these clays can block the fine drainage holes in the bag.
Fertilizer should be applied annually over the life of the plant. A good quality fertilizer containing slow and fast release fertilizer should be used. Experience has shown that an annual application of approximately three to four grams per tree for twenty litres of soil should be adequate to maintain fertility. Remember that over-fertilization can create just as many problems as under fertilization. Fertilization of irrigation water can be used, but it is best to apply a low dose rate frequently rather than very high doses infrequently.
All weed growth within the bag should be suppressed, as the roots of weeds will compete for water, nutrients and space with the crop. Mulch should not be used where it is deeper than the edge of the bag as the roots will grow through it over the top of the bag into the surrounding soil.
Fill the bag with soil or potting mix, leaving approximately two centimetres free board at the top. This aids watering and prevents roots growing over the top edge of the bag. When buried, the surface of the soil in the bag should be level with the surrounding soil, leaving the two centimetre free board above ground level. Litter which accumulates on the surface of the bag should be removed periodically to prevent roots growing over the free board.
While the volume-to-growth rate and fruitfulness relationships are the same for all species and cultivars so far tested, the absolute growth rate and size achieved will differ with each species and cultivar within a species. In any given bag volume each species or cultivar may achieve a greater or lesser size when comparing different species or cultivars. For example, the size of a Eucalyptus grandis in a 20 litre bag is considerably bigger than a cherry in the same bag. When the option is available a more compact plant is produced with spur fruiting types than with lateral fruiting types. This difference may require some adjustment in the training methods and spacing used.
Of the species so far tested, such as cherries, apples and grape plants in 20 litre bags, all should be planted on a 1.2 metre spacing. However, if an alleyway is required for tractor access, two metre wide rows could be left every third row, or have a specialized tractor to work over the rows. The two rows between the alleyways should be staggered under the alleyway system. If larger bags are used, wider spacings between the rows would be needed, e.g. 40 litre bags would be planted on a 1.5 metre square to allow for a 50% greater growth in larger bags. Some experimentation with density may be needed with species and cultivars where we have no experience at present.
EverGrow Bags can be very heavy when filled, especially if the soil is wet. When shifting or lifting bags please ensure safe lifting practices are followed, with extra people to assist as needed.
EverGrow Bags are produced in Oamaru by Evergrow Orchard
42 McLeods Rd