The large, pea-shaped seeds are sown in cold frames in Spring or Autumn. The seeds benefit from pre-soaking or chipping with a sharp blade. The plants are also available later in the season, as young plants or plugs. They are grown up canes, with the new shoots being regularly pinched out to promote a bushy habit and higher flower yields. Plants typically reach heights of 1-2m, with the flowers appearing in midsummer and continuing for many weeks if regularly deadheaded. Over 50 cultivars have gained the Royal Horticultural Society's Award of Garden Merit. See List of AGM sweet peas Pests and diseases The sweet pea plant does suffer from some pests, the most common being aphids. These insects suck the sap out of the plants, reducing growth. Mosaic virus is spread by greenfly, causing yellowing of leaves, distortion of new shoots, and inhibited flowering. A pest called the pollen beetle which is small, shiny and black, eats the pollen and disfigures the flowers. Other pests include caterpillars, thrips, slugs and snails. Another problem is mildew; this is a white powdery coating that covers the leaves and slows down growth. The sweet pea is also susceptible to ethylene in quantities produced by senescing plants. Because of this, growers are encouraged to plant sweet peas away from fruit trees among other plants prone to early dieback or senescence. Toxicity Unlike the edible pea, there is evidence that seeds of members of the genus Lathyrus are toxic if ingested in quantity. A related species, Lathyrus sativus, is grown for human consumption but when it forms a major part of the diet it causes symptoms of toxicity called lathyrism. When sweet peas are planted too close to each other it causes fungal diseases like powdery mildew. Powdery mildew looks like white-silver dust that settles on the plant's leaves, flowers and stems. Fungus suck nutrients from sweet peas and stunt their growth. In studies of rats, animals fed a diet of 50% sweet pea seeds developed enlarged adrenals relative to control animals fed on edible peas. The main effect is thought to be on the formation of collagen. Symptoms are similar to those of scurvy and copper deficiency, which share the common feature of inhibiting proper formation of collagen fibrils. Seeds of the sweet pea contain beta-aminopropionitrile that prevents the cross-linking of collagen by inhibiting lysyl oxidase and thus the formation of allysine, leading to