A - Partially weakens the argument by claiming that first time gardeners don't take adequate care of the plant, and hence get easily discouraged from buying the plant again.
B - This talk about inexpensive ways... Not relevant.
C- Demand and supply, again not relevant.
D- Strengthens claim, by arguing that plant requires natural environment to foster.
E - Weakens the argument by claiming that, the plants sold by the sellers are used to develop new techniques to help grow plants in nurseries, this would in the long run prevent the plant sellers from taking them from the wild.
So IMO E

Is caught between a rock and a hard place!