Deciduous trees and shrubs need a cold place for the winter but need no light and little water when they have no leaves and are dormant.
Apple, crabapple (malus spp.) are a very good source of bonsai material. Seedlings are often easy to find, and young commercial trees can also used. Edible apples have white flowers, ornamental crabapples have pink flowers and Siberian crabapple (Malus baccata) have white flowers and tiny fruit - particularly appropriate because the others have fruit far too big to be on a bonsai.
Birch -paper or white birch (Betula papyrifera) seedlings are found near parent trees, in damp areas, or they may be purchased. They have white bark and oval leaves with pointed tips. Bog birch (Betula glanulose) is a smaller native, with rounded, dark green leaves, found near creeks and marshy areas. Branches are flexible and easy to shape. They need moisture and are pruned in summer.
Elm - Siberian (Ulmus pumila) elms have Y-shaped trunks and small leaves with toothed edges. Seedlings are easily found near parent trees or they may be purchased as young plants. They are suitable for formal upright bonsai.
Maple - Amur maple (Acer ginnala) is the most suitable maple, with elongated maple- shaped leaves and red stems. They turn a very attractive dark orange in the fall. They are easily found as seedlings or purchased as young plants. Prune in summer only. This is a great alternative to Japanese maple.
Pincherry, Chokecherry - (Prunus spp.) include several native varieties and are also available commercially. All have attractive white flower clusters and small chokecherries. Leaves are oval with pointed tips. Pincherry has dark red bark which is pretty even when the tree is not in leaf.
Caragana - (caragana spp.) comes in many cultivars, including globe (C. frutex 'Globosa") and pygmy (C. pygmaea), which are smaller and less likely to sucker than common caragana (C. arborescens). They have small, round compound leaves. Fern leaf caragana (C. arborescens 'Lorbergi) has thin, fern-like leaves. All have small yellow flowers. These are really hardy.
Cotoneaster - (Cotoneaster spp.) is the common hedge plant in this area. It is inexpensive to purchase, especially in the spring when it is available bare-root. It's easy to bonsai, with flexible stems, ease in pruning and compact growth. Leaves are small, green ovals which turn dark orange in the fall. There is also a horizontal cotoneaster (C. horizontalis) which makes an interesting cascade.
Bearberry (Kinnikinnick arctostaphylos uva-ursi) isn't really a shrub but it makes a great cascade bonsai. It is very low-growing with shiny, dark green leaves, pale pink flowers and red fruit. It develops a woody stem, and is native to the prairies and foothills, or available commercially.
Nanking cherry, Double flowering plum are large garden shrubs commonly grown on the prairies. Nanking cherry (Prunus tomentosa) has single, pale pink flowers and edible red cherries, double flowering plum (Prunus triloba multiplex) has double brighter pink flowers and no fruit. Seedlings of Nanking cherry are easy to find. Consider these instead of Japanese cherry.
Potentilla - (Potentilla spp.) are native to the prairies and also available in many commercial varieties. Leaves are tiny and compound, and flowers on native ones are yellow; those purchased can also be pink, white or orange. Old, cow- chewed or deer-trampled specimens from the foothills can have thick, gnarled trunks that look really old.
Rose - (rosa spp.) include a large variety of native wild roses, commercially available shrub roses (particularly smaller varieties) and miniatures. The main problem is their tendency to develop spider mite indoors. They would do fine outdoors spring to fall, and possibly have their pots sunk into the ground outdoors and well mulched over the winter.
Saskatoon - (Amelanchier alnifolia) is a large shrubs which is native to the prairies, with soft, oval green leaves which turn orange in the fall, white flowers and edible purple berries. Should be easy to find growing wild.
Spirea (Spirea spp.) - includes several suitable varieties with white or pink flowers, low and bushy or taller with arching branches. These develop woody trunks when quite young and can be interesting informal uprights.
Virginia creeper (Parthenocissus quinquifolia) is a common vine which can easily make a cascade bonsai. It has five-part leaves which turn red in the fall, and is available commercially or can be rooted from cuttings.
Willow - Wolf willow and coyote willow (Salix spp.) are native to damp areas and easy to grow and bonsai. The silver leaves are very attractive. Commercially available Blue Fox willow (bluish-grey leaves) and Flame willow (orange-red bark) are also suitable. Don't try a laurel-leaf willow - they grow to 35 feet wide!
Evergreens need a winter cold but above freezing indoors, or, if native, stored in their pots sunk to above the rim in the ground and well mulched too survive. They will not survive as houseplants.
Spruce - (Picea spp.) includes many varieties such as the native white spruce (P. glauca), and commercially available Bird's Nest spruce (P. abies 'Nidiformis'), Globe spruce (P. pungens 'Globosa') among many others. There are many plants stunted by nature on mountain hillsides.
Pine (Pinus spp.) - include many native varieties in the foothills and mountains, and Bristlecone pine (P. aristata) which is the oldest known living tree in the world.
Juniper (Juniperus spp.) - include common native juniper (J. communis) and many commercially available low, spreading junipers, which make good cascade bonsai.
Larch (Larix spp.) is a deciduous conifer which loses its needles in the fall. It is difficult to transplant from the wild as it has a long tap root. Larches need moisture year round.