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).

‘ Para­dox’ sport of ’ Para­dox’

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