Quick answer
Source-bounded roll-and-merge guide: the official concept and purpose are explained, while odds, costs, recipes, thresholds, and resulting stats remain explicitly unavailable.
Mutant Plants Base Defense roll plants: the official description names rolling rare plants. Mutant Plants Base Defense merge plants: it also says merging makes plants stronger. Odds, costs, recipes, thresholds, and resulting stats are not published, so use the live roll and merge interface without guessing.
The official source confirms rolling for plants; this guide links to the merge context and omits unsupported odds and costs.
For Mutant Plants Base Defense roll plants, use the quick answer first, then check whether the topic affects a code reward, event, item, unit, map, build, value, or upgrade path.