Growing delphiniums in the plastic house this year been interesting. Summer started with an unusually warm, dry, calm November and has continued, unabated, until now. It will be one of those summers that young people today will remember as the summers that we used to have. It’s one of those summers that I will remember as a challenge.
Wanganui is generally a very temperate place with almost constant, prevailing westerly winds and our greenhouses are designed with that in mind. Long, hot, calm sunny days have therefore taken their toll. Temperatures have often exceeded 35degC in the growing houses. Now delphiniums can handle days like these providing they get some cool weather interludes – we’ve had none; well maybe a week’s worth in 5 months. This has meant we’ve had to battle to keep our plants alive - so we can fill our seed orders. Fortunately we have done that. Fortunately also, the heat is coming off now and there’s a new flush of flowers ready to pollinate.
Of course a difficult year has its advantages and you tend to learn a lot. We’ve learned to modify our growing medium and are also looking at methods of cooling the growing houses. We always knew a hot year would find us struggling and this has certainly been a wake-up call. One real bonus has been the results from a trial of plants grown in straight pumice (see images). This has been an outstanding success and something that will be of use to gardeners trying to grow delphiniums in climates normally too warm for them to survive well. Essentially the lesson is to keep the roots cool by growing them in an airy, white, inert media and supplying a total nutrient (hydroponic) feed at regular intervals. We’ll be doing more work with this to find a system that can be recommended for the home garden.
One problem with the hot weather is that delphiniums become susceptible to Schlrotinia rolfsii (see images below). This is a nasty disease which can fell a plant within days by dehydrating it. There is no cure so it is essential to remove the plant and all, but all, the surrounding soil that may contain fungal growth ... very white filaments). Treating the remaining soil with “Jeys Fluid” may help but solarisation (covering the moist soil with black plastic and leaving it over summer) should sterilise it out. This disease does not affect many plant genera (onions are susceptible also I think) so don’t worry about the rest of your garden plants. Guess why we grow our precious breeding stock in bags?
I’m going outside now to stand in the rain.