I too think denim would be a perfect choice.

You might experiment to see what kind of fabric is least likely to grab onto leaves, twigs, sand etc. Which would rule out flannel and fleece, I think.

Just an off the wall suggestion; but what about that slick windbreaker type material. It comes in many bright colors. If you put the quilt on wet grass, a couple of earthworms or a squishy bug, it would not come through. I'm not sure what it is called but it is very slick and somewhat dense.

Marcia

Denim. It comes in great colors. Otherwise, I would give her the quilt for house use and make another for picnics.
Does she go on a lot of picnics? Sound like your quilt is too lovely to go outside.

I just read this on pinterest and she used a shower curtain! Says she washes it and hangs to dry. I want to try that!

I'd look at wide home dec fabrics that come on the roll.

If you like the idea of denim, there are many options. Denim comes in lots of colors or you could use any of the bottomweights - rodeo, trigger, sportswear, etc. If you want something that has a bit of water resistant feel, look thru the babyville fabrics, They have a bit of a 'vinyly' side. Any, you could simply use a cotton and give it a finish of scotchguard to help it not soak in the grass dampness. If it is a special quilt to be used for picnics, you could also give them a same size bit of the thinnest gauge vinyl with the quilt to lay on the ground first. Roll it and tie it with a matching ribbon.

I use regular fabric for backing (usually something I found on sale) … then get a flannel backed vinyl tablecloth to put down before your quilt. Works great!

I made 2 of them for a family reunion giveaway and usesd flannel back vinyl tableclothes. They would be washable with delicate care. I used a quilt as you go method and sewed strips of blocks on to the tablecloth, then flipped it and sewed the next strip on. Then bind the edges. This keeps the moisture from coming through from the ground.

Outdoor furniture fabric comes in so many colors now. Sews like fabric, resists leaves and such like plastic. That being said, a friend just told me he uses his quilt as a yoga mat when camping because everything shakes off it so well. It is batiks on both sides, so it's just quilting fabric.