>> I used zip ties to hold down the shelves
If the zip-ties can't slip, I imagine the shelves now add some cross-bracing to the structure.
If they do slip under sideways pressure, you might try adding some waxed twine. If you "lash" two crosspieces together with waxed twine, they are bound as tightly as with glue.
Just wrapping two turns of waxed twine under each zip-tie might give good resistance to sliding or twisting. The wax gives good friction, and the zip-ties would probably clamp them down hard without any need to dig out the old Boy Scout Handbook or Girl Scout Handbook to remember how to lash things together.
P.S. I see "Snohomish County". Do you know of Steuber's Distributing in Snohomish (the town)? It's a great resource for gardeners or greenhouse people. They have a paper catalog that I browse as often as I browse seed catalogs. And they sell all sizes of pots and plug trays in onesies.
Steuber Distributing Co - - - (360) 568-2626 - - - - - - greenhouse wholesale
308 3rd St - - - - - - - - - - - - - open Sat. to Noon
Snohomish, WA 98290
