I also live in a very low-humidity climate, often under 10%, though higher during summer monsoon periods. I have been growing Vanda seedlings successfully since June. They form new leaves, elongate them and grow new roots. Previous to adopting my technique, even adults died within a week. I grow them in transparent glass or plastic jars or vases with no holes other than the mouth. I learned about this method here on Orchid Board. I will put a link to the thread at the end of this message.
I give a lot more water than most people using the vase method because my humidity is so much lower. People in high-humidity climates probably should think twice before doing some of the things I suggest.
How long is your new leaf? Martin Motes, of Motes Orchids, writes in "Florida Vanda Growing Month by Month" that well-grown Vandas should have a lighter-green band of new growth at the base of emerging leaves that is 0.3" / 1cm long. I have read people in suboptimal climates are often happy with 4 new leaves per year. It sounds like your plant is growing slower than this.
What is the temperature around your plant? They need warmer temperatures than most orchids. Days and nights in the upper 80s F / upper 20s C are perfect, and low 90s F / low 30s C are OK with enough water. Brief excursions even higher are tolerated. Growth slows down dramatically with cooler temperatures and stops with a few nights at 50F / 9C. Leaves start dropping much below 50F. If your home is in the lower to mid 70s, this alone would account for slow growth.
Do your leaves have any small longitudinal wrinkles, on either upper or lower surfaces, indicating not enough water? If they are smooth, you are watering enough. Even people living in high-humidity climates usually need to water their Vandas at least once per day if it hasn't rained, and twice or more during hot weather. Motes and others say to water until roots turn from white to completely green, which may take 2 passes with the hose or sprayer. Then, water again soon after they turn white. Mine turn white 1-4 hours after watering. I can't water more frequently than twice per day, but mine don't stay white more than 6-8 hours before I water again.
I use a spray bottle to soak the roots morning and evening. Every 3-4 days I soak them overnight in their vases, with water up to the base of the leaf fan. Then I dump out the water. Sometimes this turns into a 12-hour or even longer soak if I forget to drain them or I get called to work early. I use rain or reverse osmosis water and I fertilize with almost every spray or soak (see below.) When I went out of town for a week and they didn't get watered, they were upset, but the wrinkling went away after a couple of good long soaks. I have never had problems with all this soaking. Once I forgot to take one seedling out of its soak and left for 4 days. The roots grew very well in the water and there was no rot. I would not try this routinely for fear of water molds and bacteria.
I suspect you are watering adequately, but if you have leaf wrinkles, either soak the plants longer each day, or spray them twice or more per day in addition to soaking.
I have found humidity is the least important thing to them, if they get enough water.
Do your roots have green growth tips? Are they growing? The roots don't like being dry for too long. If so, the green growth cone on the root tip vanishes, leaving a white root. Old healthy roots are varying shades of brown, but never squishy. With tip damage, or longer periods of drying, the root tip dies and turns black. If not dead, the green root cone can regrow very quickly - sometimes after a 6-hour soak.
Every 10-14 days I do the overnight soak with 2 tablespoons / 30ml of KelpMax per gallon / 3.8 liters of pure water. I use no fertilizer during this soak in order to wash out salts. Kelp products contain rooting hormones. Since doing this my root growth has been substantial compared to before I started using the kelp. Almost every time I do this overnight kelp soak, most of my plants form new growth nubs on old roots. I am startled at how fast a fairly large root cone can form from nothing. I get the KelpMax and my fertilizer online at First Rays. I highly recommend it to Vanda growers.
What color are your leaves? Vandas are very heavy feeders. Bright light and adequate fertilizer lead to leaves a little darker than a Granny Smith apple. I would suggest measuring fertilizer with measuring spoons rather than using pinches. Motes writes that, for healthy adult Vandas growing well during warm weather and receiving adequate water, he uses 15-3-15 fertilizer about every 5th watering at a rate of one and a half to two TABLESPOONS / 23 to 30ml dry powder per gallon of water. He recommends even more for seedlings.
I use a Michigan State University formula (MSU) powdered fertilizer, 13-3-15-8Ca-2Mg. This formula is designed for rain or RO water. I fertilize at almost every watering, rather than every 5th watering, so I cut the amount to 1/5 of what Motes recommends, which works out to about one teaspoon / 5ml dry powder per gallon of water. This is somewhere around 70ppm of nitrogen in the solution. Using that amount at every watering surprises many orchid growers, but that is not quite as much as Motes recommends. I initially fed my seedlings at every watering with about 1/4 teaspoon MSU per gallon and had a little growth. Then I got Mote's book and read how he fertilizes. I upped the dose to 1 teaspoon per gallon and the plants took off growing. I suspect you aren't feeding enough.
You can find the thread on vase culture here:
Vandas in Glass Vases
I think it makes a big difference, compared to roots out in the free air. You would not have to take your plant out of the basket; just find a vase with a mouth that doesn't let the basket fall all the way in. Marshals here sells big glass vases and glass pitchers pretty cheaply. I bought some large hurricane glasses for $1.49 each at Tuesday Morning recently, perfect for seedlings with 6" roots and 6" leaf span. (This is the glass used for a Singapore Sling.) I keep water in the bottoms of some vases/jars to increase humidity at the roots, but I don't let the roots touch the water. I have found they may rot with prolonged submersion.
Stalled Ascocenda
Hybrid Mode
