Why sweet peas reward a little planning

Sweet peas are one of the most generous flowers a British garden can grow. A handful of seeds sown in spring will, by July, have turned into a fence-full of bloom, and every stem you cut seems to be replaced by two more within a week. They are hardy annuals, which means you can treat them as a fresh crop each year, choosing new colours and — more importantly — new scents.

The fragrance is the real reason most of us grow them. It is richest in the older, simpler-flowered varieties, and it drifts across a garden most strongly on warm, still evenings. Nothing else you can cut for the house smells quite like it.

The whole method comes down to a short list of habits: sow indoors, pinch out the growing tips, give them something tall to climb, feed them once they start flowering, and keep picking. Get those right and the plants do the rest.

Start them indoors — and choose your sowing window

There are two good windows for sowing in the UK. Sow in October or early November and the seedlings will build strong root systems before winter, ready for planting out in early spring. They need protection over the cold months: a cold greenhouse, a cold frame, or a bright, frost-free porch. Alternatively, sow from late January to March for planting out in late spring. Spring-sown plants catch up quickly and are often a safer bet in colder, wetter parts of the country.

• Sow into deep pots, root trainers or cardboard tubes. Sweet peas resent root disturbance, and a long root run prevents the twisted, stunted seedlings you get in shallow trays.
• Use a moist, peat-free seed compost, firmed gently but not rammed down.
• Sow the seeds about 1cm deep, one or two per pot, and water them in.
• Germinate at roughly 15–20°C in the dark. Seedlings usually appear in ten to fourteen days.
• Move them into cool, bright conditions the moment the shoots break the surface, otherwise they stretch into pale, weak stems.

Mice are fond of the seeds, so keep pots off the floor or cover them with a sheet of glass or a propagating lid. If you have had trouble before, pre-sprouting the seeds on damp kitchen paper in a sealed container works well — plant them as soon as a small white root appears. Nicking the hard seed coat with a knife or a nail file can also speed things up.

Pinch out the tips — the single best trick

When each seedling has two or three pairs of true leaves, and is roughly 10cm tall, pinch out the growing tip just above a leaf pair. Use your fingernails or a clean pair of snips. This one action stops the plant putting all its energy into a single lanky stem and pushes it into producing side shoots instead. Those side shoots are what give you a bushy, productive plant with several cutting stems rather than one.

If you are growing for exhibition, you would go further and train a single stem up a cane, removing every side shoot and tendril. For a cut-flower patch at home, though, pinching once or twice is plenty. Pinch again a few weeks later if the plants look leggy.

Give them something tall to climb

Sweet peas will scramble up almost anything vertical, but it needs to be tall — 1.8 to 2.4 metres is not excessive. A wigwam of canes, a length of pea netting stapled to a fence, or a metal obelisk all work well. Put the support in place before you plant, so you never disturb the roots later. Twine the first shoots in yourself; after that they will grip with their tendrils and climb unaided.

Choose a spot in full sun, sheltered from the worst of the wind, with soil that drains freely but holds moisture. Sweet peas are hungry plants. Fork in plenty of garden compost or well-rotted manure a few weeks before planting, and add a handful of blood, fish and bone per square metre. Space plants 20–30cm apart — closer than that and air cannot move between them, which invites mildew.

Planting out, watering and feeding

Harden off your plants over ten days to a fortnight, bringing them outside by day and back in at night, before planting out once the risk of hard frost has passed — usually late April or May in most of the UK. Autumn-sown plants can go out earlier in mild areas, but keep fleece handy.

Water thoroughly after planting and keep the soil consistently moist through summer. Dry roots are the main cause of short stems and powdery mildew, which shows as a grey-white dusting on the leaves. Mulch around the plants to hold moisture in, and from the moment the first buds open, give them a high-potash liquid feed — a tomato feed is ideal — every week or ten days. Stop feeding in late summer if you want to save seed.

Pick, pick and pick again

This is the part that separates a mediocre crop from an extraordinary one. Cut stems when the first flower on the stem is fully open and the rest are still in tight bud. Cut in the morning, using sharp scissors or snips, and take as long a stem as you can. Then go back every two or three days without fail.

The reason is simple: the plant's whole purpose is to set seed. The moment you let pods form and swell, it stops producing flowers. Keep cutting and it keeps trying, throwing out more buds in a vain attempt to reproduce. Let a single pod ripen on each plant and flowering slows noticeably within a fortnight.

Cut stems last four to seven days in a cool room if you strip the leaves below the waterline and change the water every couple of days. And do bring them indoors — the scent is far better enjoyed over a kitchen table than out in the border.