One of the interesting things about the Wisley trials is that not only do they highlight plants which are deserving of an Award of Garden Merit (AGM) but they also highlight plants which are unusually poor  and so serve as a warning to gardeners not to grow them. The Kniphofia trial, one of the season's most colourful and interesting trials, is a case in point.

Most kniphofias are intended to be propagated by division so that the plants are guaranteed all to be the same. But the trial also includes four entries of kniphofias raised from seed - and these are just not worth growing. The varieties are ‘Border Ballet', ‘Flamenco', ‘New Hybrids' and ‘Royal Castle Hybrids'. ‘Border Ballet', raised by one of the great British plant breeders, the late Ralph Hurst, and ‘Flamenco', which received an award from the European flower trialling organisation Fleuroselect, used to be excellent - but not any more.

The trouble is that over the years the quality of the stock has deteriorated so much, now the heights and colours and the flower power of the individual plants are far too unpredictable. There are twelve plants of each variety in the trial and last time I checked many weren't flowering at all. ‘Flamenco' was just one that disappointed - the picture (above, from its raiser) shows how good it once was.

The lesson: grow named kniphofias propagated by division. This is the last year of the trial so there'll be some award winners to tell you about in the autumn.

Another trial which revealed some plants to steer well clear of was the trial of begonias for pots and baskets - which I have to say is absolutely spectacular and well worth a visit. It's been sprayed against mildew but in spite of that precaution two varieties have already been removed owing to severe mildew infection. So if you're thinking of growing the new ‘Martha White' ("bad mildew, plants looking v poor" say the judges' notes) or the popular ‘Sensation Red' ("very bad mildew ") - beware: last week they were taken away and disposed of.

With this in mind, the judges decided to cease spraying the begonias and so make it easier to assess which really are susceptible to mildew and which are not. And growing almost a hundred varieties together is a severe test. We'll see how it goes...