Sowing half hardies

Cosmos ‘Fizzy Pink’

Cobaea scandens

Cosmos ‘Fizzy Rose Picotee’

Photos Sabina Rüber

Half hardy annuals are the most useful plants to have waiting in the wings to fill gaps in the border later in the summer. Flowering towards the end of July, they will often keep going until the first frosts - and as a rule they are really easy to grow from seed. Because they are slightly tender, they need heat to germinate and should be grown on in cold frames or a greenhouse before planting out in late May or even early June, after the last frost. Sow them indoors on a windowsill or in a heated propagator at a temperature of 18-21C. Here are my top five.

Cobaea (Cobaea scandens): like many half hardies, this annual climber is actually a tender perennial in its native Mexico. With large, flat seeds, it is one of those triffid-like plants that you can almost watch growing, reaching about 10 foot in one season. Also known as the cup-and-saucer vine, it produces large flowers that emerge greeny-white and darken to rich purple - there is also a white form. Grow it like a clematis on a south-facing wall on a trellis, or for an even more impressive display in a greenhouse or conservatory. Sow one or two seeds into a small 7cm pot.

Cosmos (Cosmos bipinnatus): the easiest, most rewarding annual to grow for late summer colour. The seeds germinate within days of sowing, and the seedlings grow strong and fast. Sow them one or two seeds to a module in a modular tray, and pot on into 9cm pots when they grow out of the modules (or plant straight out into the garden if the danger of frost has passed). Use them in pots or to plug gaps in a border. Don’t give them too rich a soil otherwise they will produce too much leaf at the expense of the flowers. If you find yourself with bushes of beautiful foliage but nothing else, try feeding them with a potassium-based liquid feed - a tomato feed works perfectly - and with any luck the flowers will start to form. Varieties to look out for: ‘Purity’ and ‘Rubenza’ are the classic, tall cosmos for cutting. ‘Fizzy Rose Pink’ has semi-double, pale pink flowers with darker pink edging (which I think may sometimes be sold as ‘Sweet Sixteen’). ‘Daydream’ has palest pink petals with darker pink centres. Click here for more detail on cosmos.

Tithonia rotundifolia

Zinnia elegans ‘Purple Prince’

Tropaeolum majus ‘Alaska’

Nasturtium (Tropaeolum majus): these are equally easy to grow from seed and ideal for ground cover or for climbing over a low wall or an obelisk. I like having them in my veg garden to trail around the edge of the borders and onto the paths, and use their peppery leaves and flowers in salads. Like cosmos they germinate quickly with heat. Sow them two or three to a 7cm pot and plant out the seedlings in early June. Alternatively, if the weather has really warmed up you can plant them straight into the ground in late May. Varieties to look out for: ‘Black Velvet’ is the deepest purple you can imagine, while ‘Milkmaid’ is the colour of clotted cream. ‘Just Peachy’ is in shades of fashionable apricot.

Tithonia (Tithonia rotundifolia): also known as the Mexican sunflower, this is a showy plant with burnt orange daisy-like flowers that are great for cutting. My experience with it has been mixed. It always germinates easily given some warmth, and can be grown on in a coldframe or greenhouse, but it really needs a long, hot summer and a sheltered spot to do its best. Don’t therefore plant it out until the weather and soil has warmed up reliably, otherwise it will sulk. If it has everything it needs, it can reach up to 1.8m in one season.

Zinnia (Zinnia elegans): native to South America, zinnias produce large, showy flowers in rich colours, and like cosmos and tithonia, they are fantastic for cutting. You can sow zinnia direct in early June if the weather has warmed up nicely, or sow them in March or early April in modular seed trays for an earlier show of blooms. They don’t like their roots being disturbed, so it’s best to plant them straight out from the plug trays into the garden. Once in the ground, they like plenty of sun and plenty of water, and may need staking if they get very tall. Varieties to try: ‘Queen Red Lime’ is my favourite with a colour-mix of peach, lime and red, like a summer cocktail. More detail on Zinnias here in The Flower Garden blog.

Tropaeolum ‘Just Peachy’

Zinnia ‘Queen Red Lime’

Tropaeolum ‘Milkmaid’