Can I edit the plants that are already in the greenhouse?

Although it's possible to edit plants that are still growing, unfortunately, it is not possible to edit plants that have already grown to full maturity and have been moved to a greenhouse. Plant characteristics affect the characteristics of the greenhouses they reside in, and can only be changed while they're still in their growing stages. In order to attain your ideal greenhouse, keep growing new plants with unique characteristics, and see what happens!