Chinese Elms - They Might Just be My Favorites

Chinese Elm - Ulmus parvifolia
Whenever I want to introduce someone to what I love about bonsai, I show them my favorite Chinese Elm.  If they didn't understand the attraction of bonsai already, this little beauty sells them on it every time! Ulmus parvifolia is an outstanding bonsai choice and one that belongs in every collection. It's a tree that will grow indoors, making it popular with about 90% of bonsai buyers. But it will also grow outdoors as well (up to zone 6) and when grown outdoors it shows off a unique feature.  The Chinese elm can be either evergreen or deciduous, meaning that when grown indoors or in very mild climates, the Chinese elm is evergreen and keeps (most of) it's leaves year round.  When grown outdoors and allowed to experience the fall, the Chinese elm's leaves will turn color and drop- just like any other elm. 
The Chinese elm is often confused with the Japanese Grey Bark Elm - Zelkova serrata which can be a problem because the Zelkova is marginally hardier.  I also find that the leaves on the Zelkova are considerably larger, but that might be because most of the Chinese elms I've seen for bonsai culture are grown from a variety called Caitlin, which is a smaller tree, with slightly shaggier bark, not quite as hardy and with smaller, more rounded although still toothed leaves.
Chinese elms are very forgiving bonsai trees and often recommended as one of the good choices for beginners.  Within reason I agree although some plants make a better first bonsai for the absolute beginner.  When faced between with a choice between a Chinese elm, a Ficus, a Fukien tea and a Serissa for a beginner, I'll go with the Chinese Elm every time.
If you order a Chinese elm online or buy one from a greenhouse, chances are extremely good that within a week or two after bringing it home it will start to drop leaves like crazy.  Don't worry. Keep your tree in a bright spot (not too close to a western window though - especially in summer when the light can burn anything) and continue to water to maintain an even state of "damp".  Before you know it the leaves will come back.
Grow It Indoors or Outdoors
If your Chinese Elm has been grown indoors as an evergreen all its life and you want to transition it to an outdoor/deciduous tree, you'll need to make the transition slowly. Under no circumstances should you ever put your tree outside in the middle of the winter. It must have the fall to allow the tree to harden off for the season and transition for the winter.
I keep my elms outside until they have colored and started to drop their leaves and then I move them to a cold room in the basement to rest for a few months.  Check every week to make sure they're not bone dry and about mid March (I live in Southern Ontario - climate zone 5B) bring your tree back into the light, but still keep it on the cool site.  I like growing them this way because I truly believe that the rest in winter will allow for a more vigorous, stronger tree. But that's just me. 
If you want to grow your Chinese Elm as an evergreen, keep is in a slightly cooler but still bright spot in the winter. Reduce the water slightly and cut back on the fertilizer to push for some rest.  It will drop a few leaves but nothing drastic.  Best of all if you can keep your tree outside in the summer and bring it indoors in early fall, before you need to turn on the heat you will reduce the shock of a different environment.
Chinese Elm are pretty widely available and very attractive bonsai trees and the fact that they're also pretty easy to care for is a big bonus.  I've never found them to be insect magnets although in a dry environment spider mites can be a problem.
In the winter months when your tree is likely sporting a little less foliage than usual it can be a good time to trim a few branches back and reduce some of the too many smaller branches than can occur if you tend to clip your tree to shape over the course of the summer.  A better way to manage your bonsai when it's in active growth is to trim the growth on new branches back to 3-5 leaves as they emerge (rather than shear a full tree in one sitting) but honestly, when they're growing actively, Chinese elm can be some work to keep up with.

Repot Chinese Elms and do other major work in the early spring when the tree is getting ready to put on a major flush of growth anyway. It's best not to do too much work on your tree at any one time, so if you're repotting and root pruning, you'll want to trim the upper foliage back, but don't get too carried away or you'll overstress this awesome bonsai. You can propagate your Chinese elm when you trim back branches by simply dipping the greenwood in rooting hormones and planting them in a moist medium.