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Sunday, June 10, 2012

Lupin, Lupinus spp., Fabaceae

June is the peak month for lupin flowers, although they don't seem to be such popular border plants as they once were. 

We have a family connection with lupin growing because in the 1920s my grandmother worked in Drayton Manor nursery, near Chichester in West Sussex, that specialised in growing Lupinus polyphyllus as a cut flower. This is a photo from the nursery catalogue, although the lady isn't my grandmother. The fields of flowers must have looked spectacular and I can remember as a child seeing feral descendants of these plants growing on the edge of gravel pits where the nursery once stood, and seeing them in Gran's garden. The sweet peppery scent of their flowers always triggers recollections of these stately blooms, with bees working their way methodically around and up each flower spike before moving on to the next.

Lupinus polyphyllus, which is naturally blue-flowered, was introduced from North America by the plant collector David Douglas in the  1820s and soon became a popular subject for the herbaceous border, which it graced until the Yorkshire gardener George Russell decided to try to produce plants with denser flower spikes in a wider range of colours.

Beginning just before the First World War, Russell grew thousands of lupins on his allotment and ruthlessly eliminated any that he thought sub-standard, favouring bi-coloured forms with closely spaced whorls of flowers on the flower spike. By the late 1930s he'd produced his famous Russell lupin strain and with the help of nurseryman James Baker propagated the best forms commercially from cuttings. The plants were a sensation at the 1937 Royal Horticuultural Society Chelsea Flower Show and Russell, at the age of 79, became famous, receiving an MBE and the RHS Veitch Memorial Medal for his efforts. He lived on until 1951, dying at the age of 94. Plant breeders do seem to live to a grand old age; perhaps it's the determination to see how seedlings from last year's cross pollinations will perform.

Russell probably intercrossed L. polyphyllus with other species (possibly with tree lupin and the annual L. nootkatensis) although we can't be sure because no pedigrees of cross pollinations were kept. It seems likely that he probably let bees do the intercrossing for him.

For anyone with space enough to grow the plants, breeding lupins can be rewarding , as the seedlings flower in the year after sowing. Crossing involves carefully emasculating the flowers while they were still in bud, by carefully opening the buds and removing the stamens before they can shed pollen and then, after a delay while the flower develops and the stigma matures, cross pollinating the stigma with pollen from another individual, or even another species which may or may not produce a viable hybrid seed. 

Lupin flowers, like most legumes, have a distinctive and instantly recognisable flower morphology, with a prominent standard petal at the top and a folded keel petal, enveloping the stamens and the stigma, that's flanked by a wing petal on either side.

You can always spot a lupin flower that has been 'tripped' by visiting bees because their weight forces down the wing petals so the pointed tip of the keel petal protrudes.

During the bee's visit the keel petal is pushed downwards so the stigma at the tip of its long style is forced up through the end of the keel, sweeping a plug of pollen from the anthers before it and onto the underside of the bee. You can see the pollen as the yellow deposit on the tip of the keel petal and on the stigma in this photograph. 

Plentiful pollen makes lupin flowers very attractive to bumblebees, so emasculated, cross pollinated flowers need to be covered to keep bees away until seed has set.

The most interesting lupin species that I've grown is this one - Lupinus mutabilis. It's an annual that comes from South America, where it's known as tarwi and is said to have been cultivated as a food crop by the Incas. It's seeds have very high levels of protein (40%+) and lipid (20%+) but from the horticultural point of view it's the colour and fragrance of the flowers that appeal. It's easy enough to grow provided you don't attempt to transplant it. It never recovers from the slightest root disturbance, so you either need to raise it in pots as a conservatory plant or sow it in the garden where you want it to flower.