Answer: Because your winter weather is cold, you'll want to choose the hardiest shrubs and small trees for your containers unless you can sink the containers into the ground to help protect the roots from freezing. Where temperature drops to zero and below, soil will freeze solidly and many hardy plants may be killed. This is due, not so much to cold, but to frozen soil, which does not allow tops to draw moisture, though they are still constantly transpiring. In below-zero regions, hardy evergreens, arborvitae, Japanese yew, hemlock, pines, and Douglas fir, if planted in containers with sufficient soil, will survive winters out of doors. If you want to try growing less hardy plant, you can sink the pots into the soil to protect the roots from freezing. Here's a list of some of the hardiest plants suitable for container growing:
Arborvitae. Versatile evergreen for the portable garden. Inexpensive, hardy, and quick-growing, it is ideal for hedges or background or for closing off sections. Little Gem, a variety of American arborvitae, is low and compact, a foot high, but spreading several feet.
Azaleas. Brilliant flowering shrubs requiring an acid soil. They also make good container plants in alkaline areas since soil can be prepared for them. Plants take shade, but flower better in sun. Always keep moist, since fibrous roots resent drying out.
Brooms or Cytisus. Green arching stems, with abundant flowers in spring. Require full sun and a light, sandy soil. Both the showy Warminster broom, with yellow flowers, and the familiar golden Scotch broom are dependable.
Cotoneasters. Interesting with a world of possibilities. Flowers are inconspicuous but glossy leaves and colorful berries are attractive. Rock spray cotoneaster has flat, horizontally arching branches. The small-leaved evergreen cotoneaster can be arranged around trees in planters and large boxes to avoid bareness.
Enkianthus. Handsome with small, bell-shaped flowers in pendulous clusters, fine to see close at hand. Lustrous leaves become fiery red in autumn. An acid-soil plant, requiring the same culture as azaleas.
Fothergillas or Bottlebrushes. Small shrubs with white flowers in spring and large, coarse leaves that color in autumn. Dwarf fothergilla (Fothergilla gardeni) attains three feet, but the large fothergilla (F. major) grows taller.
Hollies. Handsome plants, with shiny foliage and bright berries. Japanese holly (Ilex crenata) has dark green leaves; the convex-leaved Japanese holly has small, rounded, highly polished leaves; Haller's Japanese holly is a small, compact variety; and Kingsville is a true dwarf. Inkberry, another shrub holly, has lustrous evergreen leaves, an open habit, and black berries in fall. Leaves turn bronzy-purple in winter.
Japanese Flowering Quinces. Many varieties, including dwarfs with vermilion, scarlet, pink, rose, red, apricot, and white blossoms. These easy shrubs are primarily desired for early spring vivid color.
Japanese Yews. Among the best evergreens for hardiness, ease of culture and tolerance of sun or shade. There are upright, columnar, spreading, and low forms; all have dark green needles and are excellent for contrast with flowers. These are hardy in the North, but be sure to water all container plants in winter when soil is not frozen. The upright, rounded Hatfield and the columnar Hicks yews make good hedges. Where hardy, English yews can be substituted.
Pieris. The upright Japanese has hanging white flower clusters and bronzy-red new spring growth. The mountain pieris is lower and rounded, with upright white flower heads. Both have attractive foliage and are dependable the year-round.
Roses. Many kinds are suited to containers. Floribun-das are more floriferous than hybrid teas and can be used as low hedges or in groups. On terraces and patios include hybrid teas for color, form, and fragrance if you can face the spraying, etc. Where hardiness is questionable, store in a cool place, as a garage or closed-in breezeway, in winter. In pots and window boxes grow the delightful miniatures.
Rhododendrons. Broad, glossy, evergreen leaves and showy flowers in red, rose, pink, purple, or white. Give sun for a few hours a day for richer bloom. In winter, put in a protected spot to avoid windburning. Rhododendrons need a peaty, humusy, acid soil and plenty of water.
Spice Bush. Moisture-loving, with small, yellow, pungent flower clusters in early spring. The leaves unfolding later are large, neat, and aromatic.
Summer Sweet or Clethra. Intensely fragrant, white or pink spikes appear for several weeks in summer. Give an acid soil and plenty of water.
Viburnums. A dependable group with attractive leaves, white flower clusters (some fragrant as V. carlesi and carl-cephalum), and colorful berries. The large doublefile viburnum has flat flower heads along horizontal branches. The Japanese snowball is a showy double form.
Best wishes with your container garden!
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