I’ve tried to raise maybe a dozen Japanese maple seedlings over the past two seasons, and almost all have died. I’m down to only one left now — one just died this week. I don’t know if I’m giving them too much water or not enough. Or if they need more humidity or more shade, or some fertilizer, or what. All those cool little seedlings I grew last year died; most of them because I left them outside too long at the start of winter.
So, I posted in the maples forum at GardenWeb.com to see if I can get some advice. I’ll follow up here with their advice soon.
Here was a good reply from GardenWeb:
I have been growing seedlings for about 4 years. They do have a high mortality rate. I lose a lot of them. Weevils and slugs girdle the stems, and they get rot. You need to pot them in very porous well drained potting soil. I use a lot of crushed beauty bark and potting soil. I live in the cool PNW so the seedlings can take full sun for me. I have found miracle grow is too alkaline. My seedlings develope chlorosis using only Miracle grow. I added some acidifier to it and that helped. The growth rate is so variable depending on the type of maple. Some will grow a few inches a year and some a foot. I usually fertilize them in March or April a couple of times, and then again in July when they have another growth spurt. I usually grow them in a 4 inch pot the first year and then in late winter or early spring when they are dormant I pot them up to a gallon pot. I water the 4 inch pots in full sun almost every day. The 1 gallon pots that might be in half sun, maybe 2 times a week. It’s a lot of trial and error. I would try giving them more sun in the morning maybe 6 hours or so and see if that helps. Here are a few of mine.