Orchid Roots – Repotting

Orchid Roots: When to Repot?

As the seasons change, many orchid plants are now putting out new growth, and if you’ve done any kind of reading on orchid culture, you’ve more than likely run across the recommendation to repot your orchid plants only when new growth (especially new root growth) has just started.

There’s certainly nothing wrong with letting those plants that do put out aerial roots continue to do so; orchids that are epiphytes (tree growing) grow aerial roots as part of their natural habit. As long as you continue to water and fertilize the roots growing outside of the pot as well as those growing inside, (and, assuming the medium in the pot has not begun to decompose,) your plants will do extremely well without all of their roots tucked into some container.

However, if your potting medium has begun to decay–bark-based mediums usually have a two-year “pot life”–you will need to repot your plants to keep their root systems healthy, and the best time to do so is when you see new root growth start to emerge from the base of the plant.

But why is that timing important? Why can’t you just repot anytime you choose, as you might for other types of plants? What makes orchid roots so different, that they need special consideration?

If you unpot an orchid plant that has both aerial roots and submerged roots, the first difference you might notice between the two is the roots’ color. Since aerial roots are exposed to light, when they’re wet, they can take on a greenish-grey cast, and during active growth, their tips are often bright green or reddish-purple. An orchid’s aerial roots carry the same pigments its leaves do, and aerial roots can help your orchid plant carry out photosynthesis.

However, orchid roots that have naturally grown submerged and hidden in potting medium have no such color. Older ones are often dark brown, and newer ones are a creamy, yellow-white. They don’t carry photosynthetic pigments because such pigments are unnecessary inside a dark container filled with medium.

A difference that’s not immediately apparent, however, is the difference in velamen structure between an aerial root and a submerged root, and here is where the timing issue becomes important.

The whole purpose of velamen (the thick, fleshy, or spongy tissue around an orchid’s actual root thread) is the conservation of water. In aerial roots, the velamen’s outer epithelial cells form a thick, basically one-way water barrier. When those cells get wet, they allow water to passively enter. When they dry off, they keep the water inside from getting out. Those outer cells protect aerial roots from dessication, and only aerial roots will develop that outer wall of dead, “shield” cells to such a great extent.

In submerged roots, the outer wall is very thin, and sometimes completely absent. The plant has no need to develop an “air-water shield” on submerged roots since moisture is a lot more abundant and easily obtainable. But with no shield in place, submerged velamen is also a lot more permeable to air. It has to be to carry out gas exchange in a potted environment that’s relatively “air-poor.”

When you go to repot, and you submerge roots that have developed aerially into an environment that has a lot more water, you upset the original air-water balance that their structure has been built around when they grew: an original environment with lots of air, too much of which has to be kept out, and relatively little water, as much as possible of which needs to be absorbed. With their one-way shields designed to let water in freely but keep out drying air, water goes in, not enough air can *get* in, and the root thread inside the velamen drowns, dies, and rots.

The opposite is also true. If you take a plant that has grown its root system submerged in medium, and you decide to mount it on a piece of cork where roots that have been submerged are now exposed to constant air movement, without that shield of dead cells in place around the velamen, those roots will die from drying out, often dessicating in a matter of days: *too much* air passes in, too much water is allowed to escape.

This is why it’s best to do any kind of repotting–especially the kind that will submerge aerial roots or expose submerged ones–when the root system has just started the season’s active growth, and new roots are developing. It won’t change what will happen to the roots that are already there, but it will give the plant a chance to grow its new root structure to coincide with the environment you’ve placed it in, and the new root system can take over for the old roots that are going to die off because of the changes. Your plant will suffer the least amount of stress, and it will be ready to bloom abundantly for you once again!

So repot in time, and good growing!

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