Exotic and colourful, these quick-growing spiny plants are easy to grow.
If you consider annuals to be cheap and cheerful and a bit lacking in character then thank you for sticking with these posts – cleome is what you have been waiting for. There are many cleome species, all New World and many native to the USA ( see photo below of one I saw growing in Oregon) but the common garden cleome is C. hassleriana from South America. It is a tall plant 1m or so high with prickly stems, petioles and leaves. The plants are aromatic and slightly clammy and the leaves divided into many leaflets and pose more than a passing resemblance to cannabis (so I am told LOL). The ends of each shoot – the plants are sparsely branched – carry a seemingly endless series of flowers composed of four petals and long stamens and even longer ovary which continues to extend in length as the upper flowers open. After a while the effect is a cylindrical bottle brush with a colourful tip. Nothing else looks quite like them.
Because of their size they are not ideal for traditional bedding but they do fit well with shrubs and in the herbaceous border, extending in height and bulk throughout summer. They are raised from seed and there are mixes of pink, white and violet or single colours. In recent years the vegetatively propagated Senorita Rosalita (TM) has been introduced – it is more compact and better for containers but expensive because it cannot be grown from seed.
Cleomes are not the trickiest plants to grow from seed but they can be slow to germinate and they must be protected from frost. I used to have trouble germinating them until I learned that they need fluctuating temperatures to stimulate germination and a thermostatically controlled propagator was not helping me at all. So now I sow in early March and take them out of the propagator after a week or so and leave them in the greenhouse for a week. After returning to the propagator they usually germinate after a few weeks. The seeds must not be covered in compost – they need light to germinate. I am always paranoid about waterlogging so add lots of perlite to the compost so they can be moist but not wet. I move them to small-cell trays at the cotyledon stage and then into 8cm pots. Once the weather warms up in April they get a growth spurt and can be planted out in late May.
Remember that these are big plants. Plant them 60cm apart or more. They are best in a warm, sunny spot protected from wind if possible. Tall, airy Verbena bonariensis is an obvious partner, along with any tall cosmos or nicotiana. They also have potential among herbaceous plants to provide colour and interest after the peonies, delphiniums, poppies and lupins have faded.