From what I remember, when I started my seed bank from scratch using bogling looted plain/dull seeds, it takes 10 days to grow a plant to maturity (to the point when it can be pruned for deco or begins to produce resources/seeds). To get green thorn bearing plants, you need seeds from a minimum of 2nd generation plants - bright green snake plant or barrel cactus, which you can get from crossing green varieties of the same plant.
So, unfortunately, if they aren't provided to us, and we have to grow the resource plants to obtain those items to test, you are talking 22 days before you can even get a seed you can grow a resource bearing plant from, then another 10 days for it to mature, and another 7 to collect all the resources, so that is 39 days min to fully harvest resources from a crop of plants. On a shard that may be wiped without notice at any time, that isn't very practical. I think fertile dirt only saves you 1 - 2 days at most in the cycle, and it takes so long to get 40 dirt from earth ellies (or planting existing green thorns!) for a single pot, that isn't very practical either.
-Skylark
P.S. taking this opportunity for shameless plug: whether we get plant resources on test center or not, PLEASE CONSIDER GIVING THE PLANT SYSTEM SOME LOVE!