It seems like a good idea for many to grow they're own Kratom trees. There are some advantages, such as having a source for fresh leaves, which can be chewed. This is a treat that most people outside South-East Asia could never experience. A Kratom tree might also make a nice house plant with unique looking leaves. This is much easier said than done though. The fact that Kratom can grow up to 50 feet tall (15 meters) and 15 feet (5 meters) wide is just one of many issues. Simply trimming the tree will solve this problem, but it would drastically limit the amount of leaf produced.

Economics
Economics is a definite issue, young Kratom trees can be purchased online at costs of $40 - $80 U.S.D. per tree from some vendors. There is a wide array of information about cultivation, but many people experience a great deal of difficulty in they're attempts at growing Kratom. With this in mind, if you decide to grow your own Kratom trees, don't think your beating the high prices. With the high rate of failure, high cost of trees, and probability of weak potency, Kratom trees may be more work and money than the yield of material is worth.
As stated, greenhouses seem to produce little results with Kratom. Even if the trees grow, the potency of the material is almost always low. Most outdoor climates outside South-East Asia would not suit the needs of a Kratom tree. A tree could be placed outside, however, during the summer in many regions, while being trimmed and kept inside for the winter.

How to grow Kratom
Kratom prefers warm tropical swamp like conditions. This makes growing Kratom outdoors out of the question in most regions of the world. Green houses also are notorious for they're failure in sustaining Kratom trees. Kratom trees seem to have a high rate of failure for a number of reasons, but for many, they're interest alone is the reason for attempting cultivation. Kratom prefers humus rich, fertile, wet soils in a protected position, and is drought and frost tender. Seedlings need to be very well protected from sun and wind, but the maturing plant is quite hardy. The Kratom plant is a heavy feeder.
Indian seed distributors have sold Mitragyna parvifolia seed intentionally mislabeled as Mitragyna speciosa (Kratom) seed. This means that any seed offered anywhere in the world is unlikely to be the real thing. There is no confirmed commercial source of Mitragyna speciosa seed anywhere at this point. The best way to go is rooted clones, as cuttings do not root well and are vulnerable to infection. To deal with the fungal attacks and that clones do not root well, one should put the cutting in water with an air bubbler to increase oxygen levels; use a small amount of fungicide in the water to ward of fungus growth; change the water every day to reduce chances of fungus.
The simplest way to grow a Kratom tree would be to grow it in a normal pot and give it plenty of water. There is a wealth of information online to help with growing so read all you can. If you are serious about producing high potency leaves, it has been suggested that a wind source stimulates the production of mitragynine and the leaves. A heat source such as a reptile heating light also simulates the heat of the tropics. There are a number of factors that have been suggested to increase potency however. The fact is, it's unclear as to how and why some Kratom is more potent than others, but climate definitely plays a major role. The leaves of Mitragyna speciosa are most potent in the late autumn, just before the leaves fall off. Plants grown in subtropical or warm temperate climate are weak in late winter and spring, but potent in late summer autumn and early winter. Kratom does not need to flower to produce alkaloids. Kratom prefers high humidity. Kratom can be grown under HID lamps (HPS or MH), but be careful to keep the humidity up. After the Kratom tree is a year old, it will be tall enough to harvest some leaves. You can keep on doing this. It will not cover a daily dose, but then again, that would not be recommendable. More info about growing Kratom plants can be found in the ethnobotanicals forum.