I don't know where you are and what's available to you, but in general, look for 100% cotton.
Unfortunately, most jeans nowadays come with some amount of elastane (in order to make them fit better on a wider range of body shapes without having to do much more tailoring), so double check the materials on the label.
Stretchy jeans will last less and will hold up worse to wear and tear.
You want cotton jeans that have not been "broken in" or washed out. You also want jeans that are a heavier weight (so "summer weights" are also flimsier). Ideally when you put them on the first time, they should feel "stiff". The fibers will stretch with use, don't worry.
Also, try to wash them less often to make them last longer (I know this is for gardening, so they will probably be stained, but you don't need to wash your jeans after every use). Use less soap that you think you need, no fabric softener, wash in cold water (and hang to dry if possible). If you use a dryer, don't use dryer sheets and use the lowest temperature setting.