Are gymea lily ok to plant next an inground pool in a planter box? How big do they grow? Are the roots large

Hello Wendy, thanks for your question. Gymea Lily's are not invasive plants nor have very large root systems - they actually grow from an underground bulb. They'll perform well in the planter boxes that you have; consider regular applications of Yates Dynamic Lifter Soil Improver & Plant Fertiliser in Autumn and Spring to keep their leaves strong, green and striking. They can get taller than 2 metres in their natural environments, but in home garden settings in planter boxes, they don't get much larger than what yours are now. If any leaves start to droop down too low, to keep a neat, upright shape, you can cut away lower spent leaves using clean and sharp secateurs.