A greenhouse has limited area for growing plants, but it is not just limited to the ground surface area. Greenhouse shelving can as much as double the growing space inside without any negative affects of shade. Most shelves are designed to be mobile with wheels so they can be moved outside during good weather, and back inside at night or cold temperatures.
Greenhouse shelving generally has a mesh surface that allows water to drain completely from the plants. No pools and puddles can accumulate which promotes mold and mildew growth which can decimate plants in a greenhouse. Good ventilation always helps.
Greenhouse shelving can be a permanent installation inside the greenhouse, or they can be temporary additions for starting seedlings. Wood, cinderblocks and metal all make good stands and legs for the shelving, and wire mesh allows all the water to drain. Shelving can also help keep plants separated to prevent cross-pollination or seeding of some plants.
