The popular climbing annual, the sweet pea, belongs to the species Lathyrus odoratus. Several other species of Lathyrus are also cultivated and these, particularly the perennial Lathyrus species, will require different treatment. All the sweet peas on this site are cultivars of Lathyrus odoratus with the exception of those on the Lathyrus species page. Please note that the Winter Sunshine series require slightly different growing techniques.
Always purchase good quality seed from a specialist sweet pea seed supplier who offers a good range of the best modern varieties. Many sweet pea varieties lose their vigour over successive generations, and become weak and disease prone. Not all old varieties are affected, but stocks of 'Swan Lake' and 'Geranium Pink Improved' are now totally degenerate and bear little resemblance to the originals. Sweet pea seed should be uniform, give virtually 100% germination, and be true to type. Cheap, field grown seed of obsolete sweet peas will inevitably give poor results.
Sweet pea seed varies considerably in size and appearance. Pink and red varieties generally have large round seed, about 8 seeds to the gram, while blues and lavenders are often smaller and wrinkled, about 12 seeds per gram. There is a useful article on sweet pea seed on the lathyrus.info site.
If you have a reliable source of John Innes seed compost, or for spring sowing John Innes No 1 potting compost, these will be eminently satisfactory for sweet peas. J.I. Nos 2 & 3 are too strong for seed sowing and should be avoided. We use a good quality peat based seed compost which has given consistently good results over many years.
Germination of sweet pea seed is quicker and more uniform if the seedcoat is abraded with a small file immediately prior to sowing. A temperature of 18 - 20°c will give rapid, even germination. As soon as the seedlings have emerged, they need to be grown as cool, and with as much light, as possible.
Sweet peas can be sown direct into the open ground where they are to flower. The best time for this in the UK is mid March to early April, depending on the weather and the locality. Sow 2 - 3" apart, about 1 - 2" deep and thin the resulting plants to an average of 6-8" apart. Slugs and mice can be a real problem on a direct sown crop, so take suitable precautions before you sow.
Choose an open site which gets plenty of direct sunlight, but which has some shelter from the worst winds. Avoid planting close to an established hedge or where there will be competition from tree roots.
Young seedlings can be prone to disease if over watered, but once the crop is in full growth, a copious supply of water is needed. A well prepared site will pay dividends by having considerable reserves of available moisture. It is essential to encourage early establishment and a strong root system if high quality flowers are to be achieved.
Ideally a sweet pea plant will derive much of its nutrition, particularly nitrogen and phosphate, via its mycorrhizal symbiote. High levels of potassium are beneficial in encouraging firm, disease resistant growth but must be balanced by suitable levels of magnesium, as increasing the concentration of potassium in the soil inhibits the uptake of magnesium by the plant. For this reason heavy dressings of potassium sulphate will often lead to symptoms of magnesium deficiency.
Balanced fertilisers are safest, particularly tomato feed. Never apply heavy dressings of fertiliser to weakly growing plants - invalids need gentle coaxing back to health. Foliar feeding can be useful for plants with root problems, but frequent sprays of very dilute fertiliser will be needed to have a significant effect.
Novices are advised to chose the finest and most consistent varieties, particularly those marked (E) in our list. Our 'Exhibition Collection' of sweet pea seed comprises a well balanced selection which is equally suitable for general garden use. Plants will need to be grown on the "cordon" system, preferably from an autumn sowing.
For further advice on how to grow sweet peas, books on the subject are available
from Owl's Acre Books
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© 2007 Mark Rowland