Is it safe to use P.T.(pressure treated/ground contact) timbers when constructing a raised garden (consumables) planter box?
Well covered previously, resoundingly NO. Same for using reclaimed pallets anywhere near a garden. They are usually dunked in anti-bug chemicals. Fine to burn, not so fine anywhere else.
Onto raised beds, yesterday my son & I cleared out our old small 4x4 raised beds. I've settled on 3' max width as easier to work from either side. Even though I can still touch my toes, mostly, leaning over to get to the middle of the 4' bed was a bitch. We used cedar purchased 3 years ago. Bugs have definitely eaten into it. Not sure what but termites do look like the culprit. Even bug eaten they would last a few more seasons. That said, the new beds will be made from pine 2x10's and will each be 12' x 3' running E-W. I would imagine I will get the same life from pine as I would cedar, at much lower cost. The cost to build cedar beds the same dimension is way beyond what I want to spend. Way beyond. The nice thing about raised beds of course is that I don't have to worry about my stone filled New England yard. I bury it and laugh.
On tomatoes, this will be year 6 and I've learned a lot. Soil, sun, fertilizer, & the correct amount of water. Cheat & buy established plants too, less care, more tomatoes. The cherry tomato plant I got a Lowes last year delivered pounds of them. It got full sun (8+ hours) and was right by my front door so I remembered to fertilize 1x each week while it established in the big pot. Ones that got less sun still produced but the tomatoes were smaller and less of them. Don't use the same soil each year either. If you have 2 plots, tomatoes on one & something else on the other. Re-fertilize at the beginning with what is required for the specific plants. On water, too much leads to split skins. Still edible but they look like crap. Soil should feel just dry before you water. In July & August that is usually every few days. For fertilizer I bought Miracle-Gro for tomatoes (looks like pink sugar) and it worked well. Mix a tablespoon in a gallon of water and go.
Back to guerilla gardening, do you know your terrain? I mean, have you really walked it in day & night to see what is there? I live in a residential 'burb ~30 minutes out of Boston but it is just amazing the amount of hidden & unclaimed spaces you can find just by walking around. Spaces that never or rarely get seen by anyone, even the dog walkers. Figure everyone on the M-F, 9-5 schedule has the same focus each day and isn't looking outside of their routine. Much of the space therefore is never seen because it is never looked for. I'd bet within 150-300' from your place you can find a number of places to grow that are covert. Just don't plant more than you can care for on (1) trip. Assuming you do this, each time you visit you want to pack in everything you'd need such as enough water. If you have a huge garden you won't be able to care for it in one trip and will have to go back again & again. Repeated trips to one spot will attract attention. Better to have multiple spots and a weekly rotation. Not that I've put much thought into this......