Why you should sow biennials now

For Sissinghurst's creators, constant experiment and renewal were part of the life of the garden. Vita and Harold experimented all the time with shapes and colours and moved whole groups of plants to better places, rejuvenating the garden every season with new varieties or fresh ideas.

Since the National Trust took over the garden’s care, the range of biennials used here has expanded to more than 20 types, varied each year and most of them grown in a large, open bed, west of the Rose Garden. We grow several varieties of some, such as wallflowers, and always in sufficient numbers to ensure a generous flowering display.

The time to sow is when you’re removing this year’s plants, typically at the end of May and into June, depending on the year. However, there are subtle timing differences that matter: the first you should sow are Sweet Williams (Dianthus barbatus), followed by foxgloves, verbascum, wallflowers and pansies last of all, perhaps as late as July.

All of our seeds are sown into peat-free compost with added perlite. The compost is an excellent fine grade and the addition of the perlite provides an open, free-draining medium for the seedlings’ young root systems. Small, shallow pans are used for sowing and each is clearly marked with the sowing date and plant name.

Spacing the seeds and sowing evenly will help prevent issues with overcrowding and poor growth when the seedlings emerge; the size of the seed determines the depth of sowing. A good rule of thumb is that the sowing depth should be roughly the same as the size of the seed itself. Apart from digitalis and verbascum, which need light to germinate, all of our biennials are sown in our cool pot store in the dark, with the majority of seeds germinating within two weeks.

When they’re large enough to handle, we prick off the seedlings into plug trays before lining them out in the biennial bed for summer. By the time we’re ready to plant within the garden, usually around late October, the majority have made good strong plants.

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We had good success this year growing wallflowers as separate colours; in the Cottage Garden, we planted Erysimum Blood Red, Fire King and the long-lasting orange Siberian wallflower, Erysimum x marshallii in distinct groups rather than mixed like a fruit salad. We also experimented with tulips planted in blocks or drifts, behind or adjacent to the wallflowers rather than scattered through them. We found such schemes worked best where the most strident colours were represented less frequently; say, in a ratio of 1:4.

Equally successful has been the white foxglove, whose vertical accents have contrasted in shape most effectively with the more horizontal effects of the autumn-sown umbellifer Ammi majus.

The great challenge when growing biennials is deciding how to avoid a ‘gap’ once they have finished and what should follow them. I employ various techniques: replacing with a late-sown annual, such as cosmos or nicotiana that has been pot-grown to a good size and so fills the space instantly, or replacing with a fast-growing tender perennial which, again, fills the space instantly (such
as dahlia, canna or salvia).

Another method is to have plants nearby that can sprawl over and through the decaying biennial-
things such as rhodochiton, lablab or clematis. Finally, my preferred option is to interplant with an annual or tender perennial while the biennial is still in situ, giving the replacement plant more time to settle and grow.

Many other biennials can be easily grown from seed, including the impressive Angelica archangelica, Eryngium giganteum, Isatis tinctoria, the indispensable Lunaria annua, clary, caraway, Salvia sclarea L. var. turkestanica, Glaucium flavum with delightful pale-grey foliage, the iridescent Anchusa azurea and the under-rated Campanula patula. In addition, some perennials are strongest as young plants and can be grown as biennials-such as Althaea rosea, Lupinus polyphyllus or the night-scented hesperis.

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