I would suggest that your Cannas are suffering from the English summer (lack of it). Originating from warm, humid climates I would say our early summer is a very dodgy time for all but those that have been proven the hardiest of Cannas. My recall is that we used to plant them out fully mature and only when the sub-tropical bedding was laid out in late June to early July. Of course this will vary according to the county location.
My way would be to keep them in the glasshouse or a very sheltered but sunny place until July to obtain the best looking foliage.
I know that some varieties and in some areas can be left in the ground permanently, but even so, the goodness of the foliage is dependent upon our best summer climate.

I will be interested to read others views.

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Cannas require large amounts of water to grow well and the brown edging sounds like they have been crisping in the wind. If they are growing then water and feed reqularly. Lots of both.

This has not been a good start to the season but the canna will still push on if given good conditions.

I grow all my plants in pots and the best ones are the plants sitting in 1-2 inches of water at all times, regardless of variety.

There still seems to be a lot of old myths regarding their hardiness, but while they will not take frost, they will still grow regardless. Cannas prefer hot and sunny conditions as do I, but will make do with what is available.

If grown in greenhouses, they should be left there as they grow tall and soft whereas outdoor grown plants while somewhat shorter, will be much stronger.

My cannas (2000+) have all been outside since they were potted at the end of March. Nighttime temps regularly dropped to 3 or 4 degrees above freezing almost till the end of May. They do look a bit tattered here and there, and are only about half the height they should be but by the end of this month I expect to have good results.

The weather in Bucks probably has not been much different but if given a handfull of growmore once a month with daily watering, Your plants will also thrive.

Moriati

My cannas have been outside, standing in the pond for almost a month now. The "Durban" types are about a foot high, but a "Stuttgart" tuber I bought this spring is still an unfurled spike 6" high. The only brown bits are last year's stump which is still attached. I'll remove them when I get round to it.

I didn't grow them for years as they didn't seem to do well, but I think I had a virusy one which died in the end, a merciful release.

My latest ones did great last year, stood in the pond all summer, no food, no need to water. They bulked up really well, so I've bought a few more. I got the original plant from a car boot sale.

I did buy a water canna, "Tany", from B&Q, which I placed in the pond according to strict instructions, never to be seen again!

I'd particularly like a "Panache" canna after seeing it at Wisley last summer. I like the slender, tall types and the coloured leaf types, not the lumpy salmon types.

I move the entire pots into the greenhouse (more or less frost free, but only just)in about November. I don't think they're desperately tender, there's other plants much more nesh!

I have Cannas which stay in the ground all winter. No losses from the cold, but I might end up killing them myself. Not enough sun, not enough heat, not enough water, or all three, means that they aren't exactly blowing my socks off.