Two Februaries ago I received a bunch of D. scorpioides gemmae and they sprouted nicely last year. They didn't produce gemmae of their own, nor did they flower. This summer they did flower and shortly afterward, one by one, most of them died. I have several cultivars of pulchella, nitidula, carberup, badgerup, etc... and they all flowered as well - but the plants didn't die. Any ideas why?