Home Gardener's Propagation (UK Only). David Squire

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      Biennials and herbaceous perennials

       Are these long-term plants?

      Biennials are plants that flower in their second year of growth. In gardens, they are sown in spring and early summer for flowering during the following year, when they are usually discarded after they cease flowering. However, many biennials are actually perennial in nature, and will continue growing and reseeding themselves. Herbaceous perennials, once planted, have life-spans of 3–5 years before they become congested and need to be divided.

      WHEN SHOULD I SOW?

      You can sow both hardy biennials and herbaceous perennials in seed beds outdoors, later transplanting the young plants to their growing positions in beds and borders. Their sowing and transplanting times differ slightly, however.

       Hardy biennials

      • Sow seeds in drills in a nursery bed in spring and early summer. Always sow seeds evenly and thinly.

      • Thin seedlings slightly when they are large enough to handle. If necessary, refirm soil around those that remain. Then lightly but thoroughly water the soil.

      • In autumn of the same year, or early spring of the following one, transfer plants to their growing position.

       Herbaceous perennials

      • Sow seeds in drills in a nursery bed in spring and early summer, later thinning them to wider spacings and subsequently planting them in a nursery bed. During spring or early summer of the following year, plant them in borders. After planting, thoroughly water the soil.

      • Alternatively, sow seeds in shallow pots in spring and early summer and place them in a cold frame; this encourages more rapid germination and prevents compost becoming excessively wet, causing seeds to rot.

      • A further choice is to sow seeds in seed-trays (flats) in spring and early summer and to place them in a cold or slightly warm greenhouse. Later, plant them in a nursery bed.

      SOWING BIENNIALS AND HERBACEOUS PERENNIALS

      1 During winter, dig a seed bed in a sheltered part of a garden and remove perennial weeds. In early spring, use a wooden rake (or a large metal type) to level the surface and break it to a fine tilth. If the soil is stony, do not rake off all stones as they help prevent compaction.

      2 To firm the surface evenly, shuffle sideways over the entire surface. As well as consolidating the soil, this helps to break down large lumps. Do not use a garden roller, because it makes the surface uneven. Additionally, if the soil is damp its use results in clods of soil on the surface.

      3 Use short canes to mark the ends of each row and stretch a garden line between them. Then, use the edge of a draw hoe to form a shallow drill, 6–12 mm (¼–½ in) deep. Remove the garden line before sowing seeds. Take care not to knock soil into the drill.

      

      4 Sow seeds evenly and thinly in the bases of the drills. Congested seedlings are susceptible to diseases, and this also creates unnecessary competition for water, air and nutrients. Stand on a board to avoid compacting unsown soil.

      5 Cover the seeds either by straddling the row and shuffling forward to guide friable soil over the seeds (above), or by using the flat top of a metal rake to push and pull soil over the seeds. Firm the surface by using your feet or the flat top of a metal rake.

      6 Label the ends of the rows and lightly draw a metal rake over the rows (in the direction of the rows and not across) to remove footprints and level the soil’s surface. This prevents water resting in puddles on the surface.

      Biennials to raise in this way

      • Alcea rosea (Hollyhock)

      • Bellis perennis (Common Daisy)

      • Dianthus barbatus (Sweet William)

      • Digitalis purpurea (Foxglove)

      • Erysimum x allionii (Siberian Wallflower)

      • Erysimum alpinum (Alpine Wallflower)

      • Erysimum cheiri (Wallflower)

      • Myosotis sylvatica (Forget-me-not)

      Herbaceous perennials to raise in this way

      • Acanthus mollis (Bear’s Breeches)

      • Acanthus spinosus (Bear’s Breeches)

      • Achillea (Yarrow)

      • Campanula (some species)

      • Centaurea (some species)

      • Dictamnus albus (Burning Bush)

      • Echinacea purpurea (Purple Cone Flower)

      • Incarvillea delavayi

      • Leucanthemum maximum (syn. Chrysanthemum maximum; Shasta Daisy)

      • Lupinus polyphyllus (but not named varieties)

      • Sisyrinchium

      THINNING SEEDLINGS

      Thinning seedlings provides the ones that remain with more space. If left, they become congested, thin and weak.

      • Thin seedlings as soon as they are large enough to handle.

      • Do not leave pulled-up seedlings on the surface of the soil.

      • Check that soil is not loosened around remaining seedlings; if necessary, firm soil around them.

      • When thinning is complete, lightly but thoroughly water the soil’s surface. This may need to be repeated.

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