One often reads that to successfully grow trees in containers, one should re-pot the trees frequently in to progressively larger containers, such as 3 gallon, 5 gallon, 7 gallon, and so on. I think that's just too much work. After all, the greenhouse is crowded, and it would a real chore to have to pull potted trees in and out all the time. It would be much easier to just plant the tree in the container that will be its permanent home, hook it up to the irrigation system, and leave it there indefinitely. How does the tree know if it's in a 5 gallon pot or a 20 gallon anyway? I did a little research, and the conventional wisdom is that the trees must be re-potted frequently because as the organic components of the potting soil such as compost and peat break down they compress, and cause the roots to suffocate or even rot. In addition, pots with a lot of dirt hold a lot of water, and excess water can also lead to root rot. However, I also read that the same rule does not apply to container media that are largely inorganic, such as those primarily composed of expanded shale, turface, or crushed stone. Trees planted in such a mix could be placed in a large container right away because they drain very quickly, and the loose structure of the container media wouldn't bread down over time.
There is a recipe for such an inorganic-based potting media called "Al's Gritty Mix", which consists primarily of calcined clay and crushed granite. While it drains exceptionally well (maybe too well for Houston heat), it is also fairly expensive and labor intensive to make, and very heavy. I don't think I could possibly afford to make it, and I know I wouldn't be strong enough to safely remove a tree from a large pot filled with the stuff. Fortunately, in my search for a less expensive alternative, I stumbled upon an interesting idea on the Citrus Growers Forum. Millet, one of the moderators, had been experimenting with using decorative cedar mulch as a significant component of potting soil. The cedar mulch is very cheap ($3.19 for a 2-cubic foot bag), resists rot, and repels pests, and is slightly acidic, all qualities that should make it an ideal addition to a container mix. Others on the forum had tried the mix and felt comfortable recommending it, so I decided to give it a try when planting my mangos.
JRN sells used plastic pots in all sizes ranging from small to huge. I bought several of the largest they had (45-gallons), and then picked up eight bags of Pro-Mix Mycorrhizae (a peat and vermiculite blend) from Plants for All Seasons. Finally, I picked up eight bags of cedar mulch from the neighborhood orange big box store, brought it all home, and got to work. I set out the monster pots, and into each I mixed two bags of cedar mulch (2-cubic feet each), and one bag of Pro-Mix (2.8-cubic feet each). I can tell you that mixing up that much material is very hard work. Once it was all mixed well, I watered it down, and transplanted the mango trees. I'll hook them all up to the irrigation system this week, and hope to see some good growth once the weather warms up again. A few of them already have some blossoms forming, but I'll need to pinch them off. Mango blossoms are especially susceptible to anthracnose and other fungal diseases. To combat this, I rigged up some small fans to increase the air circulation, and I'll be ready to treat any sign of infection with neem oil should they appear.