Trespassing and vandalism (if you hit the sprinkler systems). If you know what the bank is, write and get their permission.

Squatting is generally illegal. Plus gardens take time investment and you are very likely to lose the effort and cash that you put into it when you are thrown out rather abruptly.

I suggest you try vacant lots that are NOT in foreclosure. Contact the owner and offer to pay $1 a year to garden on the site. The owner gets the benefit of not having to keep the weeds under control.

If you do that, you will have to purchase liability insurance that covers both the gardeners and the land owner. Those policies are pretty cheap if there is no building.

The benefit is that you know you can garden until harvest, and if you take good care and look neat, you might get to keep the use for a couple of years, since bare lots are virtually unsalable in this economy.