Anyone with a windowsill has access to a potential miniature garden, to grow decorative or useful plants close to hand, come rain or shine. Use windowsill boxes with drainage holes, and fix them firmly in place in case of high winds. Potting soil is preferable to garden soil, which is heavier and more prone to compaction. Fill your windowsill with miniature forms of garden perennials, annuals and bulbs, and even shrubs.
Woodland perennials grow well in partial shade conditions on windowsills. Persian violet (Cyclamen hederifolium) blooms profusely from late summer through fall, bearing pink or white 2-inch flowers. Ivy-shaped, variegated foliage 4 to 6 inches high grows soon after flowers begin to fade, and keeps windowsills looking attractive until spring, when primrose (Primula vulgaris) flowers appear. Blooming in clusters, the pale yellow blooms rise on stalks 6 inches high above a basal rosette of crinkled leaves. Persian violet is suitable for U.S. Department of Agriculture plant hardiness zones 5 through 9, and primroses are hardy in USDA zones 4 through 8.
Windowsills are convenient for growing annuals from seed, within easy reach for watering and tending to young plants. Wild pansy (Viola tricolor) sometimes survives for two or three years, or may reseed itself in the window box if dead flowers aren't removed. It bears small, pretty flowers that look like little faces, in a range of colors including yellow, purple, white, lavender and blue, and grows up to 6 inches tall and wide. Annual phlox (Phlox drummondii) bears clusters of tubular rose-red flowers on plants 6 to 12 inches tall and wide. Cultivars with white, pink, red, purple, buff or lavender flowers are also available. Both plants bloom through the growing season in cool climates.
Bulbs on windowsills provide a sense of excitement in spring as shoots push through the bare soil. Tazetta daffodil "Minnow" (Narcissus "Minnow") and Siberian squill "Spring Beauty" (Scilla siberica "Spring Beauty") provide an early spring display of creamy white, yellow-cupped daffodil flowers and contrasting deep blue Siberian squill blooms. "Minnow" grows 6 to 8 inches tall and is hardy in USDA zones 5 through 9, and "Spring Beauty" grows 3 to 6 inches tall and is suitable for USDA zones 2 through 8.
Dwarf evergreen shrubs provide permanent structure in windowsill planting boxes. Holly "Rock Garden" (Ilex "Rock Garden") is a dense shrub with deep green glossy leaves. In 15 years it grows only 12 inches tall and 22 inches wide. Hardy in USDA zones 5 through 9, it can be pruned in winter to maintain an even shape. Boxwood "Compacta" (Buxus microphylla "Compacta") is another dense, slow-growing shrub, growing only 8 to 10 inches tall and wide in 15 years. Bearing light green small leaves, "Compacta" is hardy in USDA zones 6 through 9. Both shrubs grow best in partial-shade sites sheltered from cold winter winds.