Sowing flowers for next year in May/June is a very cost-effective way of boosting flower numbers and reducing the expense of buying plants next year. With nearly a full summer to grow, sturdy plants with vigorous root systems develop to be planted out in autumn for a spring and early summer display next year. Some biennials can be bought as plug plants in late summer which saves time and space.
Plants sown now can be true biennials that grow one year and flower the next, typically being discarded after flowering. Some are actually short-lived perennials – foxgloves and hollyhocks are typical examples. Other very tempting subjects include Campanula pyramidalis, Canterbury bells, Eryngium giganteum, Iceland poppies, Siberian wallflower (Cherianthus allionii), stocks (Mathiola incana) and sweet Williams. Some highly ornamental wildflowers are worth considering; Scotch cotton thistle (Onopodium acanthium) and weld (Rededa luteola) for example.
Wallflowers are also technically biennials – although often considered bedding plants – as unlike pansies or viola they don’t flower until spring. The same goes for bedding daisies (Bellis), Rudbeckia hirta and forget-me-not, but the latter gets tangled and mildewed if sown before July.
If you fancy raising pansies, polyanthus, primulas and viola, sow in a slightly shaded spot.
Other plants are true perennials such as delphiniums and lupins. In these cases they are both so susceptible to intractable mildew – and for lupins, aphids – that there is a good case in many gardens to discard them after their first flowering. Seed is not expensive. However, in other cases – once you have good plants – softwood cuttings can be taken of the best individuals so many years of plants can be obtained for the cost of a packet of seed.
Popular favourites include Angelica archangelica, aquilegia and oriental poppies. Alpine or rock garden plants such as aubretias, alyssum and spreading dwarf campanulas can also be grown as biennials. In fact, it is worth trying any perennial seed you have now – most are likely to succeed even if not classically sown in mid-summer.
Many perennials can be sown in early spring to flower the same summer, but this needs greenhouse space and is less reliable than overwintering. Annuals sown now are likely to flower rather poorly and prematurely so await September before sowing most of these for spring flowers.
Ideally sow in a seed bed of bare but reasonably sunny ground, perhaps in a corner of the vegetable garden. However, sometimes there is space in borders to sow where they are to grow next year. Rich soil leads to delicate plants so moderate fertility is sufficient, perhaps where compost was added to a preceding crop. These plants typically resent cold, wet soils, so where clay is involved, raised beds will be helpful. Sow in rows to aid hoeing. Sow as shallowly as possible, 10-20mm, into moist ground, watering the night before if necessary, in rows 15-20cm apart.
Alternatively, biennials can be sown in cell trays, and either overwintered in these or potted up in summer. These are easier to defend against slugs than open ground, but overwatering must be avoided – especially over winter.
Sturdy, stocky plants are needed, and to avoid leggy weak ones, sow thinly, and in July, replant seedlings further apart or pot up into small pots as is convenient. Allow the young plants 15cm in rows 25cm apart, watering before and afterwards if the weather is dry.
Pot up plants sown in containers or cell trays – 7cm pots for smaller plants, 9cm for lupins and other large plants. Feed every fortnight until late August and water carefully aiming for strong plants for autumn planting.