Moisture retention layer on top of leca - Orchid Board

GinaStarSeed · #1 08-20-2017, 08:24 AM

HI. I am wondering about the dry layer on the top of he Leca. The layer can be up to an inch thick in mine that remains very dry. I had a thin layer of spagnum moss on to, however, have removed it due to hearing it can break down and clog the system. Also, I fear it's to moist for the base of the new shoots, although he new roots seem to like it. What do others do to hydrate that top dry layer? It seems to be dehydrating for the new roots at the top layer. I know I could mist them regularly on the top; however, I am looking for a more handsoff option. I thought about a rock layer, but upon trying, it seems the pebbles want to fall into the leca. Thanks for helping and sharing what methods you use. So appreciate help in this process.

Orchid Whisperer · #2 08-20-2017, 09:26 AM

I have very few orchids that want/need a layer at the top of the pot to preserve moisture. Some Pleurothallids, Bulbophyllum, Coelogyne come to mind.

Unless you live in a desert, for many other that kind of moisture preservation may not be needed and could potentially be harmful to the roots.

What are you growing? Answering that question could get you a better answer, since not all orchids are alike.

Ray · #3 08-20-2017, 09:58 AM

There are two, competing processes going on that affect that:

The ability of the medium to wick water upward, and
The evaporation rate of the water.

The first is a fixed entity, determined by the physical characteristics of the LECA you are using, and the height of the LECA above the reservoir. Evaporation, on the other hand, is controlled by your ambient humidity, temperature, air movement, and even light intensity.

If wicking > evaporation, the LECA is wet all the way to the top. If wicking < evaporation, it's not, and the degree of difference determines just how high or low that "dry line" is in the pot.

If you are unable to alter the environmental conditions, placing an evaporation barrier in place to slow it can help, but the choice can be an issue, as you have learned. One decent idea I've seen was to make a round, clear plastic "cover" out of a vinyl page protector sold at Staples and the like. Make the disk be about 1" smaller in diameter than the pot, cut it radially to the center, and remove a portion in the center, about an inch bigger than the base of the plant. The gaps allow you to water with it in place, and it still allows air flow, but some of the evaporating water will condense on the plastic and drip back into the pot.

As OW stated, that really is not an issue for established plants, whose roots are below the surface, but it can be an issue when newly emerging roots come in contact with dry LECA, as it can desiccate them, stopping growth. An occasional misting of the medium can remedy that, though.

Also, keep in mind that the moisture content is a gradient, not 100% saturated below the "dry line" and 100% dry above it. Just because it doesn't appear wet, does not mean there's no moisture in it, so that desiccation issue might not even exist.

In other words, stop worrying about it. If you see a root-tip issue, try to increase the moisture in the LECA. If you don't, forget about it.