Yes, you can use pressure treated wood for a DIY urban planter box, but with a few important caveats. Modern pressure treated lumber, produced after 2003, uses alkaline copper quaternary (ACQ) or micronized copper azole (MCA) instead of the older chromated copper arsenate (CCA). These newer treatments are considered much safer for garden use because they contain no arsenic. However, they do contain copper, which can leach into the soil in very small amounts.
For flower planters, herbs, or ornamental plants, pressure treated wood is perfectly fine and will last significantly longer than untreated wood—typically 10 to 20 years outdoors. The copper levels released are far below what would harm plants or soil microbes.
If you plan to grow edible vegetables, especially leafy greens or root crops that absorb more minerals, you might want to take extra precautions. Lining the planter box with a thick polyethylene plastic barrier can prevent direct soil contact with the wood. Alternatively, you can use cedar, redwood, or untreated pine with a natural wood preservative for a completely worry-free option.
In short: for urban planters with flowers or non-edibles, pressure treated wood is safe and cost-effective. For vegetable gardens, use a plastic liner or choose untreated rot-resistant wood.