James currently works full time as sole gardener in a large private family garden in Hertfordshire. As a teenager, he took an honours degree in Landscape Management at The University of Reading, with a gap year spent working on trials for the RHS at Harlow Carr in Harrogate. He has worked as a gardener ever since taking on a wide range of jobs, varying from landscaping new-build properties, taking care of stately home estates, running his own garden maintenance business and most recently growing, assessing and writing up the trials of fruit, veg and flowers for Which? gardening magazine.
Sweet peas are some of the most popular annual flowers to grow from seed. If you grow them well you will be blessed with armfuls of blooms all summer. I have grown trials of sweet peas for the RHS and National Sweet Pea society at RHS Harlow Carr in Harrogate and for Which? Gardening magazine at Capel Manor Gardens in Enfield. Here are a few top tips I learned from some of the countries top growers on how to get the best results from your sweet peas.
1. Choose the right variety
Sweet peas can be divided into two main groups – ‘Grandifloras’ and ‘Spencer’ types. For the expert, flowers of grandiflora types have fused keel petals, Spencer’s do not. For everyone else, Grandifloras tend to have a stronger scent so are really useful if you want that quintessential sweet pea sent in the house. ‘Hi scent’ (pale pink) and ‘Cupani’ (purple and maroon) have some of the strongest scented flowers. If you want the really big, frilly flowers with long stems for exhibition, then you will be better off with a Spencer type. A couple of my favorites are ‘Chastworth’ (pale blue) or ‘White Frills’ (white). More recently, some sweet peas have been bred to combine the best of both of these types and are called ‘Modern grandifloras’. One of the best all-rounders for both cut flower and scent is ‘Albutt Blue’ (pale blue). If your seeds have a solid color, they will produce a solid colored flower, if they are striped, the flower will be striped too.
2. Sow them in the Autumn
Sweet peas are very cold tolerant and only need a little bit of warmth to get them started. During October, sow them 1cm deep in a good quality compost. If they are kept at around 16° C for 7 to 14 days you will soon start to see shoots. Sweet peas are a delicacy for hungry mice so make sure they are covered with a propagation lid or sheet of glass, to exclude any rodents until you start to see shoots appearing. Sweet peas that have been sown in the autumn are much stronger so will start to flower several weeks earlier than plants sown in the spring. Seed production used to be done in the heat of California, meaning seed casings were very tough and needed to be chipped to help them shoot. It is no longer necessary, and you are more likely to damage the seed than help it.
3. Grow them cold
As soon as you see the shoots are poking through the compost get the plants outside. Sweet peas are incredibly hardy. Even in November the temperatures aren’t too cold for them and they will quickly ‘harden off’. Keeping them in cool conditions with plenty of light will ensure they don’t get too leggy. Keep them in a sheltered spot away from the worst of the winter storms and as soon as the weather warms up in the spring they will start to branch out. Pinch out the tips to stop them getting leggy and weak.
4. Use ‘root trainers’
Sweet peas have surprisingly deep roots so will appreciate being grown in deep pots. You can use a tall plant pot or specialized cell trays called ‘root trainers’. These are trays made up of individual deep, narrow cells that unfold to allow easy planting without disturbing the roots. They also have ridged sides to encourage roots to head downwards.
5. Plant them in fertile soil
Sweet peas will always do better if they are grown in a well fertilized, moisture retentive soil. In the Autumn before planting, incorporate plenty of organic matter in to the soil where they will be grown.
6. Train your sweet peas
Sweet peas will grow rapidly and will need some support. Wig-warms or A-frames built from canes will give the plants something to scramble up. If you keep tying them into the supports regularly it will ensure all of the shoots stay heading in the right direction and the flower stems will be much straighter for cutting. Exhibition growers grow their plants as a ‘cordon’. As the plants grow all the side shoots and tendrils are removed, leaving just one growing tip on each plant. This focuses all of the plants energies into producing just a few top quality blooms every week. Once the plant reaches the top of the support structure all the ties are cut and the stem is laid horizontally along the ground (layered) and the tip bent up the adjacent cane to continue training. Untrained plants will produce more flowers, but with curvier stems.
7. Keep them deadheaded
You will need to pick the flowers regularly during the season. Aim to visit the plants at least once a week and cut every flower that is open. Removing old flowers means the developing buds will mature faster, and if old blooms are left on they will soon start to divert energy into producing seeds rather than flowers.
8. Watch out for aphids
Sweet peas are prone to attacks from aphids in the late spring whilst they are growing most rapidly. Keep a close eye on your plants for the first signs of any clusters of aphids at the tips. Aphids are easy to control with a suitable spray, such as a soap solution, or wiping them off with the fingers.
9. Prevent mildew by regularly watering
One of the other problems that can cut short the display from sweet peas is powdery mildew. It appears as a fine powdery dust on all the leaves and stems and will quickly spread, causing plants to grow slower and possibly stop flowering. This is most likely to take hold during hot, dry spells in July and August. If the plants are kept well watered they are less likely to be affected, but always water at the base of the plants and do not spray the leaves, as this can increase its spread.
10. Clean tools between plants to prevent transferring viruses
Unfortunately sweet peas are prone to viruses. If plants become infected they will appear paler, less vigorous and any flowers may show unusual streaking and discoloration. Remove any plants that show signs of sickness to stop it spreading and clean any tools that may have sap on them with disinfectant.