For the last few weeks I have been pulling out handfuls of ‘nuisance’ plants that have popped up as seedlings. Too afraid to add them the compost heap, lest they perpetuate in years to come, I have filled up the green bin with pincushion scabious (the wilding blue Knautia arvensis and the maroon-red K.macedonica), mallows galore (mostly the blush-white Malva moschata) and countless meadow cranesbill (Geranium pratense). These thugs have smothered my choice plants, many of which are now lost, and I only have myself to blame. For when it was horribly wet last summer I hid inside and didn’t get out there to deadhead. Consequently I now have a forest of unwanted seedlings. It’s made me appreciate my sterile plants: the ones that can’t set seed. Being sterile, they flower for much longer. Sometimes it’s due to being too petal-stuffed to let pollinators in, sometimes it’s not having proper reproductive organs and sometimes it’s incompatible pollen. The doubles generally have extra petals that replace stamens etc. These full flowers are highly prized by plant breeders, but less so by insect life and pollinators. Many double roses can’t sustain a bee.

Generally the best compromise is to have some long-lasting doubles, like Rose ‘Buff Beauty’, and some singles or semi-doubles like ‘Bonica’.

Several hardy hybrid geraniums are sterile, probably hybrids between closely related species. These hybrids often have extra vigour and some of the best sterile geraniums take up lots of space. The best three are the mid-blue ‘Rozanne’, the black-eyed magenta ‘Patricia’ and the darker blue ‘Orion’. ‘Rozanne’ (voted Plant of the Century at the Chelsea Flower Show this year) was found growing in the garden of Rozanne and Donald Waterer’s garden in Kilve in Somerset some 24 years ago (1989).

It is a very similar plant to Geranium wallichianum ‘Buxton’s Blue’ to look at, sprawling in habit with a similar flower. However, the flowers are larger and appear in summer, not in autumn. ‘Rozanne’, which also has G. himalayense as the other parent is probably the best of the three. Like many sterile plants it is impossible to propagate traditionally and it didn’t become available until it was micropropagated by Blooms of Bressingham, who launched it at the Chelsea Flower Show of 2000. Another identical plant, ‘Jolly Bee’, bred by Macro van Noort, was also in the trade. It was withdrawn from July 2010 after a costly seven-year court case ruled it to be the same plant. Bees still visit ‘Rozanne ‘despite its sterility, so ‘Jolly Bee’ was a good name. The deeper-blue ‘Orion’ (a ‘Brookside’ seedling found by Dutch nurseryman Brian Kabbes in 1990) is similar to ‘Johnson’s Blue’. It came to prominence on the RHS Wisley trial of hardy geraniums held between 2002 and 2006. ‘Orion’ begins flowering in May, has a rest in July when it should be sheared down, and then goes on until late. It’s tall and billowing and it hates drought. I lost several plants in 2011 when my garden was positively arid.

The black-eyed magenta ‘Patricia’ is my third stunner, a deliberate cross made by Orkney plant breeder Alan Bremner between G. endressii and G. psilostemon. This one is not for the faint hearted: it’s bright and brash and I love it for its ability to do well.

Most long-flowering plants are sterile and they include the early-flowering wallflower Erysimum ‘Bowles’ Mauve’, the lovely Jacob’s ladder Polemonium ‘Lambrook Mauve’ and the frilly orange Geum ‘Prinses Juliana’. The latter will flower for 14 weeks, but a couple of years ago I planted the even better, paler ‘Totally Tangerine’, bred by Tim Crowther of Walberton’s Nursery. This has come through wet summers, hard winters and cold springs to produce long wands of flower in early summer and these persist for many months.