What is most often overlooked with Sweet Peas, is that different strains have been developed that have the ability to flower at different times of the year. The original Sweet Pea comes from Sicily and this must have over twelve hours of daylight each day before it is able to flower. The majority of cultivars retain this characteristic and all English Exhibition Sweet Peas are Late or Summer Flowering.
Winter Flowering strains have been developed that will flower with just ten hours or less daylight each day. These are especially valuable to cut flower growers as if sown in late summer (February- March in the Southern Hemisphere) and grown under glass or plastic, they will flower during the winter months when prices are at their highest.
There is a big difference in growth habit between the Winter and Summer flowering types. Winter flowering types tend to produce a single main stem that rushes up and produces flowers. This characteristic is accentuated, as when sown under warm conditions, the plants grow very quickly.
In contrast, Summer flowering types have traditionally been sown in the Autumn in areas with cold winters. Small plants go through such winters with little growth and only start to move when things warm up. In this type the main stem tends to be overtaken by strong basal shoots and the plants become bushy, in contrast to the rather spindly Winter types. The problem is that if Summer Flowering types are sown in the Autumn in areas with mild winters, they just keep growing until the days are long enough to allow them to flower, by which time the plants can be very tall. Even worse, if sown too late when the days are long, they are unable to flower until the following summer.
Over a century ago Spring Flowering types were developed which will flower with eleven hours daylight. These became popular with home gardeners in Australia, as plants were able to flower from late winter before temperatures became too high. A tradition of sowing Sweet Peas on St Patrick's Day (17th March) arose and despite New Zealand's different climate, the tradition crossed the Tasman Sea. This is far too early for Summer Flowering types in Northern New Zealand.
Spring types tend to have the stronger habit of the Summer types, but flower much earlier. Winter types perform well if sown later (May-July) and develop stronger plants, albeit still with a single main stem. With these differences in flowering response, it is possible to have Sweet Pea flowers throughout the year. It is just a matter of working out a growing protocol that suits your area.
Spring and Winter types sown November - December will give you blooms in the Autumn