That pretty much depends on which animal you want to keep out since different ones dig to different depths. The usual method is to dig a trench 1 foot deep and 1 foot wide and put the fence fabric, bent into that backwards "L", into that trench. A fence fabric with a 2 x 4 mesh would keep out a gopher but not moles or voles so that too is determined by which animal you want to keep out.

Egg,

I agree with Kims description of underground fencing ( mine is under whole veggie garden (16 yr old son helped a lot!)), but I will also add to it. If Groundhogs (woodchucks) are the primary culprtit (varmint), then you want to add an overhang at the top of the fencing too, g'hogs are great climbers, and will climb over the top of a straight fence.

dadgardens,
how is the fencing placed under the garden? to what depth, and how did you do it? have you had problems with froundhogs and has this stopped it? i'm also in the hudson valley.
thanks,
egg

egg,

My biggest problem is with the deer( 18- 30), but the groundhogs(2-8) are a close second.

My 17 yr old son helped me dig up a 4' section of the garden about 18" down and then we placed the wire mesh, and covered it with the soil we had moved. We then dug up the next 4' stretch layed the mesh and covered it, third 4' width was the same. We used 6' foot fencing (2" by 4") for the vertical (above ground) fence, and bent the top 6" down to parallel the ground, veggie garden looks like moderate security prison (not maximum-no razor wire) if you are up close, but the green plastic covered mesh is nearly invisible at 75'.
Yes, we stopped the groundhog, but only after bending the top of the fence outwards horizontally, the fat demonspawn did climb it before that (twice!).

I had tried using a shallower, underground layer of fencing outside the raised bed, but he just dug a longer tunnel; he wanted my veggies and was willing to work hard to get them.

As an aid toward keeping deer out - the garden has internal fencing (trellis' and beanpoles) which seem to deter Bambi's kin- - - they don't seem to like narrow spaces, if they have to jump in, beanpoles help by providing more "clutter" for their eyes to deal with.

One other unusual use for fencing (hardware cloth actually 1/2" by 1/2" mesh), involves burying it an inch or two above crocuse bulbs (or other small bulbs) that are attractive to chipmunks/squirrels/other rodents.

Don

to follow up, i installed a 1" mesh fence 1' below ground, and 1' bent outward into an L shape. i did this two summers ago, and it's been great. no animals have breached it, and i haven't lost anything i've planted. the fence is 3' above ground, attached to my 2"x4" main fence, which is 6 ' tall. it was much easier to do than i'd feared.