In the right environment, many annuals, perennials, herbs and some shrubs self-sow. Native plants are the most likely candidates to do this, but annuals and perennials whose flowers have been left to dry on their stems also drop seeds. When these seeds make contact with soil, they put down roots and provide plants for the next garden season. Use self-sowing plants with caution; many can be invasive.
The list of annuals that self-sow is large. Forget-me-nots (Myosotis species), cosmos, spider flower (Cleome species), silver dollar (Lunaria annua), love-lies-bleeding (Amaranthus caudatus) and moss rose (Portulaca species) are some of the most prolific self-sowers.
The long seed pods at the base of the delicate-looking cleome burst open in fall, dropping round black seeds that germinate easily. Plants can be moved in spring. Cosmos species and cultivars are grown for their large open-, cupped- or bowl-shaped flowers on long stems with fernlike foliage. Both cleome and cosmos prefer well-drained soil and a sunny location. Both plants grow to 5 feet tall, but neither needs staking. Shorter varieties are also available.
Perilla frutescens, also called the beefsteak plant, is a member of the mint family. Originating in Asia, it is as invasive as its mint cousins. Perilla is a decorative herb with purple to red-tinted leaves that last through summer and fall. Attractive to butteries, the plant prefers moist, well-drained soil and sunlight. The plant is sometimes confused with dark basil and coleus because of its leaf structure and colour.
Native to the Mediterranean, fennel (Foeniculum vulgare) is a hardy perennial. Both an herb and a spice, fennel has an aniselike flavour. Seeds can be used whole or ground and stored in airtight containers out of direct light. Fennel leaves are used with fish and to flavour breads, cakes and candy.
Borage is an annual with bright blue star-shaped flowers. Its leaves have the flavour of cucumber. The herb attracts bees and is often grown as a companion plant to vegetables because of this. Borage likes full sun and grows from 18 to 36 inches tall. It blooms from late spring into midsummer.
Hellebore, bleeding heart, black-eyed Susan, wild columbine and coral bells self-sow freely. Hellebores come in green, yellow, deep purple, dusty purple, pink and mottled shades. They are among the earliest flowers to bloom, starting in late winter, and sometimes have flowers that last through fall.
Bleeding heart (Dicentra spectabilis) is an early-spring bloomer with pendant heart-shaped flowers that come in pink, white, red and yellow. Most species and cultivars prefer moist, fertile, humus-rich neutral to slightly alkaline soil and a shaded location. Foliage, which is susceptible to leaf miners and yellowing midsummer, may be cut back to the ground when it becomes unsightly.
Black-eyed Susans (Rudbeckia fulgida) have orange-yellow daisy-like flowers with a conical brown centre. These late-summer bloomers prefer a heavy but well-drained soil and grow in full sun or partial shade. They also have a long vase life.
Three of the most notorious shrubs for self-sowing are Japanese barberry (Berberis species), burning bush (Euonymous alatus) and various Spiraea species. Japanese barberry, which comes in green, yellow, rose and deep mahogany, has barbed spines between its leaves and bears fruit in the fall. The shrub self-sows so readily that it is on the invasive species list in several states, as is burning bush, a deciduous green shrub that turns brilliant red in autumn.
There are more than 80 species of deciduous and semievergreen spiraea, which grow best in fertile, well-drained soil in full sun. Spiraea japonica is a diverse and colourful species that readily propagates itself.