Siberian elm (Ulmus pumila)

This can be an ideal learning tree for novices. From late March until late August throughout the Colorado Springs area, small wild-growing specimens of this with trunks as thick as your thumb or larger can be collected with a few lateral roots and potted up. You can even use a mixture of up to 75% "ant gravel" -- the large sand grains found piled up on ant hills -- plus 25% the soil the tree was growing in. ("Ant gravel" needs to be lightly sifted or even rinsed in an old-fashioned fine weave collander or sieve box to remove most of the dust particles. During the first few weeks you'll probably notice various sprouts in the soil mix as seeds the ants collected germinate -- just pick them out as they get larger without any harm to the elm.) Look for short and stout, possibly repeatedly-mowed plants, or find a taller specimen with very good lower branching. You can cut the top off that one and regrow the apex. Unlike evergreens, you can safely bare-root elms and cut off the bottom of the tap root if necessary as long as you have several side roots intact. Place either outside in a sun-protected area or next to a south or west-facing window indoors so it gets full sun a few hours a day. Be sure the soil mix doesn't dry out. Within two weeks you'll start to have a new crop of leaves and buds on the branches and trunk. Carefully pick off the wilted older leaves. If the tree is growing outside, you can now give it more direct sun. The more sun the tree gets, the smaller the new leaves will open up to be. Practice trimming and learning about bud and branch-growth directions. Rotate the pot a quarter turn every few days. Several smaller-trunked specimens can be trained together as a mini-forest. And very small elms can be made into mame bonsai -- just don't let the soil completely dry out.

It is said that since the Siberian elm is a pioneer species in nature, it makes a less than desirable bonsai because it has a tendency to lose branches through die-back. Years of effort designing a specimen could be foiled because of this. It is also very invasive, so be sure to properly dispose of discarded specimens and cuttings.

On the positive side, though, specimens can be collected into the end of August here, especially those that can be dug or pulled up fairly easily. Thus you have a few months to scope out the best looking leafed-out wild growing plants. With sufficient rootage, the collected trees will rebud within a few weeks after dropping their current leaves. As wild trees sometimes show extensive leaf-skeletonizing, simply remove these unsightly leaves on a newly-collected specimen, give the plant's foliage a good washing shower of plain water, and wait for intact leaves to now appear. You can also carefully pinch off most of the original leaves when a collected specimen is potted up in order to help speed up the regrowth process by slowing down water loss through transpiration.

"The tree has considerable variability in Dutch elm disease resistance. Moreover, like many other elms in North America, it is highly susceptible to damage from many insects and parasites, including the elm leaf beetle (Xanthogaleruca luteola), powdery mildew, cankers, aphids, and leaf spot However, it is the most resistant of all the elms to verticillium wilt...
"The species has a high sunlight requirement and is not shade-tolerant; with adequate light it exhibits rapid growth. The tree is also fairly intolerant of wet ground conditions, growing better on well-drained soils. While it is very resistant to drought and severe cold, and able to grow on poor soils, its short period of dormancy, flowering early in spring followed by continuous growth until the first frosts of autumn, renders it vulnerable to frost damage.
"As an ornamental U. pumila is a very poor tree, tending to be short-lived, with brittle wood and poor crown shape, but has nevertheless enjoyed some popularity owing to its rapid growth and provision of shade... In the UK, its popularity has been almost exclusively as a bonsai subject, and mature trees are largely restricted to arboreta...." (per Wikipedia)

SEE ALSO:

Bonsai4me Species Guide, http://www.bonsai4me.com/SpeciesGuide/Ulmus.html
Bonsai Forest of Siberian Elm, http://dunbar.blog.com/2007/04/02/bonsai-forest-of-siberian-elm/
Bonsai Sashi-no-eda Siberian Elm Development, http://sashi-no-eda.blogspot.com/2007/05/american-elm-development.html
Floridata species care guide, http://www.floridata.com/ref/U/ulmu_pum.cfm
Gardening.eu species care guide, http://www.gardening.eu/arc/plants/Bonsai/Ulmus-pumila-L/71281/