The science of sweet peas
Thanks to the work of Dr Keith Hammett, New Zealand is the centre of the world when it comes to the development of this fragrant bloom.
Dr Keith Hammett explains how new breeding breakthroughs are achieved.
kiwis love to celebrate international success, especially in rugby and other sports. I reckon Dr Keith Hammett deserves similar accolades for his worldleading plant breeding exploits.
Recognition certainly came his way in 2013 when one of his sweet peas was chosen as the flower to represent the centenary of the Chelsea Flower Show. The Royal Horticultural Society (RHS) selected one of his strains from trials held at Wisley Garden and named it ‘Chelsea Centenary’, undoubtedly one of the greatest horticultural honours bestowed on a Kiwi.
Keith regards plant breeding as an art form, his motivation the same as that of a composer or painter. He begins by visualising his objective, and then he formulates a plan as would be expected of a scientist. His inspiration comes from his love of the plant and his belief that it can be improved. Financial reward is a secondary benefit.
Sweet peas have been worked on by generations of breeders; Keith compares himself to a competitor in a relay race who runs a leg then passes the baton on. He feels fortunate to have Dharini Marinkovich working with him to continue his work if he “drops dead”. Although still in her 20s, Dharini has developed a close understanding of the breeding operation after two years working with Keith.
Keith initially set out to breed sweet peas with bicoloured flowers, as these had largely disappeared because judging criteria in sweet pea shows dictated that only strains with flowers of one colour (known as selfs) won prizes. Exhibiting
had become a competitive sport with arbitrary rules resulting in the demise of bicolours, so Keith set out to rectify this.
The development of selfs had been, in fact, some achievement as the original wild sweet pea was a bicolour, as was the very first cultivar ‘Painted Lady’.
For his breeding programme, Keith returned to the ancestral ‘Grandiflora’ strains that are both bicoloured and strongly fragrant. Their exquisite fragrance has been inherited by strains such as ‘High Scent’ and ‘High Society’ which are the most popular varieties in the Hammett range today.
Although his goal of bicoloured sweet peas was achieved, Keith was still not satisfied with the result. He figured this was because their colour intensity was the wrong way round – the wings, which are the closest part of the flower when viewed, being paler than the standards. Keith explains that in a landscape, the closest mountain is darker and more defined than the furthest mountains that fade into the distance. So Keith set out to develop reverse bicolours with wings darker than the standards.
It took 20 years to produce reverse bicolours ‘Leading Light’ and ‘Blue Mountain’, but still, their colour contrast was not yet sufficiently distinct.
Keith sought a breakthrough, but his challenge lay in working with a crop that, in more than 300 years, was based entirely on subtle variations within just one self-pollinating species, Lathyrus odoratus.
To put this into historical context, it had taken breeders a century to produce the first six cultivars, and it was only by the end of the 19th century that breeders had sufficient varieties to produce the slightly largerflowered ‘Grandiflora’ types.
The single most important breakthrough in sweet pea history happened in 1900, when a seedling with large flowers and
Many sweet pea breakthroughs have come via natural mutations – aka sports – where a seedling offers a desirable new quality (like the paler blue sport of ‘ Paradox’ below).