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Graham Rice on Trials

Updates on trials and awards from the Royal Horticultural Society by Graham Rice

Dahlias in containers

Posted by Graham Rice on 28 Oct 2009 at 11:29 AM

Dahlia 'Spanish Conquest' in the Wisley trial of dahlias in containers. Image: ©GardenPhotos.comThis year's dahlia trial at Wisley has been unusually fascinating and I'll be reporting on a number of its features over the next few weeks. Picking up on recent trends, this year twenty one of the entries were grown as individual plants in pots to assess their value as container plants.

Immediately one thing is obvious. These plants, of course, are smaller than most dahlias - if they were 4ft/1.2m tall, as are many varieties, they'd be far too large for most containers. So we tend to look down on the flowers from above. But many varieties hold their flowers so they face sideways - ideal on a tall plant which is viewed from the side but the impact is much reduced when you look down at the flowers on a short plant from above.

‘Spanish Conquest' (above, click to enarge) was one whose flowers are more upward facing and which made a real impact. The colour is wonderful too, with red buds opening to old gold flowers with burnt orange centres. The plants were prolific and seemed to cope with dry conditions better than many. I thought it the best of all those grown in containers this year.

Dahlia 'Keith's Pet' in the Wisley trial of dahlias in containers. Image: ©GardenPhotos.comOn the other hand there was ‘Keith's Pet'. This is a lovely dainty little dahlia with neat dark purplish foliage and bright white flowers with just a hint of pink showing through from the backs of the petals. But the flowers face sideways and unless you get down on your hands and knees - as I did to take the photograph - you never get the full impact of the flurry of flowers.

Other outstanding varieties in containers were ‘Exotic Dwarf', a single yellow with upward facing flowers, ‘My-nute Blend', a very prolific red and yellow double, and also ‘Gallery La Tour', with upward facing pink and white double flowers over dark leaves.

There were a couple of other points that came out of this trial of container grown dahlias. There was no watering system set up, no drip nozzles so that the pots could be easily watered at the turn of a tap. The result, I'm afraid, was that with pressure of work meant that the watering can just didn't come out often enough and many varieties would have been a great deal more impressive if properly watered.

And as we assessed them we also noticed the compost in the pots. It had shrunk so much that there was a large space between the tops of the pots and the compost. We concluded that the peat-free compost had already started to decompose and so had shrunk - not good for the plants. The begonias hated the peat-free compost this year as well. Sometimes, nothing but peat, or at least a good proportion of peat, will do.

Of course the varieties that could cope with these problems really were impressive.



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