distilled water vs. RO water

what is the difference between them?

Both methods are used to remove dissolved solids (ions) from water. In basic terms, the distillation process boils the water in a still, and the steam is condensed out and collected. Any of the non-volatile components that were in the water are left behind, so you end up with water whose mineral content is usually less than 10 parts per million.

Reverse Osmosis forces water through a filtering membrane. The pores in the membrane are small enough to filter out a lot of the molecules that make up dissolved minerals, but it can't filter out all of them.

Between the two methods, distillation will give you water with the least amount of dissolved solids.

As far as using either type on orchids, there's a huge debate about whether it's really necessary if you have good quality tap water.

If you decide to try one or the other, you may have to add back many of the dissolved minerals (Calcium, Magnesium, Potassium) that were taken out: orchids (and a whole bunch of other living things) need them in trace amounts for sustenance. Fertilizers that contain trace elements will more than likely take care of that.

Our tap water here is very good, so I don't use any kind of filtration before the tap water hits the plants. So far, I haven't had any problems.

My tap water was leaving a residue. When I looked it up on-line at the Water Co, I found that the water is very alkaline. Someone from Lexington, Kentucky told me that it has a lot of lime in it. Anyway, this residue can't be good, right?

That's exactly the kind of situation that an RO unit is designed to fix. Unless you're growing calcicolous paphs that thrive under the extra load of minerals, hard water that leaves deposits is usually considered too alkaline for other types of orchids.

If you only have a few plants, bottled water (not necessarily distilled) works fine, but the dollars can add up quickly if you have a sizeable collection. If you're getting whitish calcium deposits on your plant's leaves, you can get rid of those by wiping the leaves down with milk or pineapple juice.

Because distilled water is so "mineral-poor," it will tend to ionize any minerals it comes into contact with, and some growers have reported that the distilled water actually "robbed" calcium from their orchids' tissues. I don't know whether this is true or not, (I haven't done any experimenting with it) but I do know that people who use distilled successfully usually have to add a calcium supplement in before using it. (That can be as simple as pouring 5 gallons of the water into a bucket and throwing in a handful of crushed oyster shell, then letting that sit for a couple hours before using it.)

An RO unit may be exactly what you need--but they cost as much or more as a good metal halide or high pressure sodium light system....

Hmmm. Doesn't sound like there are any good options right now. I'm investing in a self-made light cart so that is the budget for right now. I was tempted to try distilled but sounds like that might actually backfire. Pray for rain!