They are few questions in gardening that have simple answers. Most involve a fair amout of ‘well it depends…’. None more so than the question ‘how will this do in a pot’. Well, it depends.
Ok, so some back ground information. Pots are a great solution for a number of problems. They allow us to grow things where there is no ground to grow in, they are mobile, and they can keep plants contained. All of these are good aesthetic reasons to use pots, but they are not good horticultural reasons.
Simply put pots are, generally speaking, a harsher growing environment that being planted in the ground. Pots (and therefore the plants roots), heat up, cool down and dry out faster than the ground. If drainage is blocked, pots can also become saturated faster and remain saturated for longer. Pots do offer protection from competition from other plants, and to some extent soil-born pathogens. However, pots also restrict root growth, and this can stress the plant and make it more prone to disease and pests.
If a plant is only to be in a pot short-term, then these constraints are much less important. Living in a pot long-term however is another matter.
It is possible to grow plants and trees in pots for many years, but there is an increasing amount of maintainence that needs to be done to keep plants happy and healthy. The key is to understand what happens to the growing media. Over time, what ever the plant is grown in slowly deteriorates. As it does, the capacity of the soil to hold moisture and, therefore nutrients, is diminished. Added to this, each year root growth increases, so the proportion of soil to roots changes. The result is more and more roots trying to get ever increasing amounts of water and nutrition from soil that is both decreasing in bulk and in its capacity to supply what the plant wants. Also, as the roots grow and age, there are less room for the fine feeder roots to occupy. Eventually the plant will end up looking sick and weak, no matter how much water is given to it or how often it is fed. There simply isn’t enough capacity in the soil to supply the plant what it needs.
The only way to curtail this downward spiral is to renew the soil and prune the roots. Root pruning works just like branch pruning – it encourages new growth from dormant buds. However, it’s important to remember that the plant has to have sufficient root system to enable it to continue to live whilst the new root growth is happening. Normally a root prune will be done at the same time as a branch prune. The ratio should always favour roots ie; if 20% of the roots are pruned, then more than 20% of the branches should be pruned. A tree can recover from losing a large amount of canopy, but is much less able to recover from losing a large amount of root structure.
At the same time as the roots are pruned, old soil can be gently teased away from the remaining roots, and new soil added to the pot. The best potting mixes to use are those that contain a large amount of coarse organic material, that will break down at a slower rate.
All this maintenance means that some thought needs to be given to how the tree is planted in the pot. In most cases it is best not to plant the tree directly in the outer pot. By that I mean that if you are using a decorative terra cotta pot for instance, plant it into a plastic pot that is slightly smaller than the terra cotta pot. Alternatively, if you do have to plant it into the outer pot, it is a good idea to line the inside of the pot with flexible plastic, such as pond liner. This means that when you need to lift the tree out to prune the roots and replace the soil it will be much easier, without the danger of the roots being stuck to the side of the pot. Remember to make sure that holes are punctured through the base of the liner to allow drainage. The holes in the liner should line up to the drainage holes in the pot. An old trick was to place pieces of old, broken pots over the drainage holes. As pots are concave, the pieces of pots sit over the holes but don’t block them. This stops the potting mix falling through the holes, or them getting blocked with roots or pieces of organic matter. It is also a good idea to seal the inside of any porous pots with a waterproof sealant. Even polyurethane varnish will do the job. This reduces evaporation through the pot.
With care, it is possible to maintain plants in pots for a considerable time. Possibly the “oldest pot plant” in the world, a South African cycad (Encephalartos altensteinii,), was collected in South Africa in the early 1770s, and brought to England in 1775. It is 16′ (4 m) from the base of the stem to the growing point.