31 out of 5 stars 13. Shelves are adjustable and can be placed anywhere on the support poles in 1 increments. Harbor Freight Greenhouse Shelves Greenhouse Shelves Harbor Freight Greenhouse Greenhouse Garden Grow Greenhouse Shelving Unit Steel 3Tier Staging Storage Outdoor Racking. Greenhouse shelving units . Sold exclusively by AmazonBasics this 4-tier plant grow rack comes in both black and chrome and is priced at just under 40. Most greenhouse shelves will be made out of metal as this is a sturdy and practical material and is easy to move. Adding high level shelves to your greenhouse not only increases capacity by as much as 50 in your greenhouse but it also gives you a higher storage space for items that are best not watered regularly such as your seeds and tools. Wood plastic and wire are also materials that are used to manufacture shelves but are a lot less common. Giantex Outdoor Portable Greenhouse Multi Tier Shelves Stands Small Shelving Green House for Herb and Fl