“Our plants aren’t looking too good,” astronaut Scott Kelly tweeted from the International Space Station on December 27, 2015. He was right: The attached picture showed four baby zinnias bathed in magenta light. Three of the four leafy stalks were discolored and curling in on themselves. The station’s garden was struggling to recover from a mold problem. It’s an issue familiar to terrestrial gardeners. And while on Earth, the problem means a trip to the local nursery for replacements, in space you can’t do that.