The phrase "mature plantings" refers to fully grown or fully developed plants that have reached their complete size and have been growing for a longer time period. Full definition
You can buy more mature plants from a nursery or plant the seeds after the last major frost of the season. (rivertrailahandpetlodge.com)
They're the beginnings of full - grown plants, and often contain slightly less or the same nutritional value as the whole mature plant they might have grown up to be! (thenakedlabel.com)
Most important, harvesting plants just at the peak of their growth would add greatly to biomass production because mature plants generally show little net growth and harvest of CO2. (issues.org)