My experiments thus far have yielded very poor results, with any dahlia except the harlequin mix. I have about a 25% success rate with Black Beauty, and a 0% success rate with both the Fireworks mix and Giant Dahlia from Johnny's. The Giant Dahlia has also been a flop in my garden, slow to grow, slow to flower, and the flowers like to turn brown around the edges and fall apart.
My container dahlias that die seem to get root rot/pythium. I may be over-fertilizing and over-watering, as far as dahlias go. It may also be that container dahlias prefer a lighter pro mix, possibly with more perlite. If I tried it again with the varieties that died on me, I would mix my pro mix about 50/50 with perlite and see if that makes a difference.
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