If the space were owned by a charitable organization, the money left by each individual would be a charitable gift reduced by the fair market value of the food taken, and the proceeds would not be taxable income for the organization because it is tax exempt.
As an individual, treating it as income subject to a deduction for cost of production (not including your labor), but with the loss not being deductible as there isn't an intention to profit, would probably be the correct treatment as that treatment is what is usually what is applied to a "pay what you can" arrangement that apart from the price, resembles a business transaction.
If no money were collected, it would probably be characterized as a gift by you to the persons taking it, which if less than $16,000 per person per year for all gifts made to that person, would be exempt from gift taxation.
You could argue for bargain sale treatment, with the value of the produce taken net of the money left as a gift, and that would be a potentially arguable position if the money left is less than the value of the produce taken. But this characterization would probably not hold up if the amount of money left exceeded the out of pocket cost of growing the produce, in which case treating it as a low value sole proprietorship would be much more defensible than treating the money left by people as a gift to you.
Generally speaking, when one person makes a gift and another person makes a counter-gift in a related action, treating each donation in isolation as gifts is not an appropriate tax treatment. I'll see if I can find authority, but, of course, cases with such tiny amounts in controversy almost never result in formal appellate decisions or regulatory action, so any binding authority wouldn't be four square on point factually, which matters in the law where reasoning by analogy and logic often doesn't produce the legally correct result.