Container growing:
Ever since man has wanted to grow plants he has tried to grow them in containers.
The success of this growing method depends on 4 things:
--the species:
--the growing medium:
--the container:
--the nutrient requirements
All of these ingredients are interelated.
Most plants are designed to grow in an unrestricted environment, whether it is in the ground or perched in a tree, as in the case of epiphytic plants such as ferns or orchids.
Recognising the natural habitat of the species concerned gives a good indication as to the right growing medium for the species and also the right container, since the container often will modify the properties of any growing medium.
Drainage is the most important aspect of container growing and it is the drainage/aeration properties (the ability to shed water as well as supply air to the root zone) of a medium that are most affected by the choice of container.
Drainage is affected by the particle sizes in the medium and the proportions of each size or stucture dictate how readily water will creep upward or move downward due to capillary action.This can be referred to as the capillary column. Finer particles allow water to creep further up into a medium against the force of gravity, so that water can literally hang in a growing medium. Water will move downward through a medium in response to gravity from coarse particles to finer particles very easily until it is held in place by capillary action. If it reaches a barrier such as the bottom of the pot or some of the old fashioned "crocks", then the layer of water that will be suspended in that particular medium will sit there. The old fashioned "crock" idea actually impedes drainage as do pots that do not sit flat on the ground since this breaks the capillary column. Polythene growing bags actually provide the best capillary contact with the ground, and therefore the best capillary or drainage column, as long as it is sitting on sand or another medium with similar capillary properties to the growing medium. If the planter bag is sitting on coarse gravel the capillary column is broken and a layer of water will hang in the bottom of the bag. (someone should design a rigid pot with a flexible bottom to give good basal contact to maintain the capillary column)
Shallow trays such as traditional Bonsai trays therefore can become quite wet if the growing medium is quite fine. Since all growing mediums break down with time and as this happens the drainage and aeration get worse, it is best to repot each spring and replace at least 1/3 of the volume of the medium with fresh material.
Once the physical aspects of the growing medium have been ascertained, then the nutrient requirements can be addressed. This again is species dependant. As an example many species of proteacous plants dislike phosphates. These plants have been grown successfully by adding a type of clay that locks up phosphates to the growing medium , but this means a change in water requirements, since clay alters the structure markedly.
Each different growing medium has a different ability to hold nutrients. Peat has some ability to hold nutrients, but granulated pine bark has practically none. The ideal nutrient supply comes from slow release coated fertilisers which release nutrients in response to medium temperatures and moisture content. When using these products be sure to take note of the longevity and actual nutrient content of the product, since these factors will indicate when and what supplementary feeding is required.