I read the following from an article on so-called‘urban farming’：
Natalie Jeremijenko, an aerospace engineer and environmental health professor at New York University, came up with a rooftop farm design to solve these common problems for urban farming. Her fixtures may be more economical than other urban farm concepts because they take up real estate that otherwise goes unused, and unlike other urban farm designs, they can pack in the plants, because everything, from the integrated systems to their bubble shape, is a slave to efficiency.
What does the phrase ‘they can pack in the plants’ mean in this context，please？