The initial question asked about a specific plant substrate, but there has been no info as to the intended plants, fish, water parameters, etc., so I will simply offer a general comment.
If you have fish intended (as opposed to just an aquatic plant tank with no fish) I would not use any so-called "plant" substrate, under a cap or sand or not. Some of these certainly have no benefit at all. Most if not all can be detrimental to fish in one of several ways, from roughness (I had this with Flourite years ago) to bacterial issues to ammonia poisoning. Consider the fish, provide the substrate they need (some must have smooth sand, and this is still without any question the overall best aquarium substrate). It is easy enough to add nutrients if required via liquid fertilizer and/or substrate tabs.