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We have been getting lots of big spikes on our Gladiolus, 2 or more feet long, but unfortunately, the cut spikes are frequently flopping over in the vase before the blooms are spent (stem is folding over).

Any suggestions as to what may be causing this, and how to fix it?  Are some varieties better than others in terms of stronger spikes?  Or is it likely that there is some environmental cause (sunlight, nutrition, etc.)?