Modified pattern to knit in the round on DPNs:

Cast on 26 sts over one needle. (28 for a larger one)
First 2 rows: k1, slip 1 with yarn in back.

Divide stitches on 2 needles, alternating the stitches, to open the pocket. Slip half of the sts from each needle onto another needle (7 sts each needle), and start working in the round.

1st Round: * k1, m1, k to end of 1st needle, k to last st of 2nd needle, m1, k1. Repeat from * across remaining 2 needles. (30 sts, 32 for larger)

Knit until the pocket is 4” deep. Cast off the first half of the stitches, and work the remaining sts back and forth for 1” (I knit the flap in garter st.).

Add the ties.

Since I live in a high desert environment, I was afraid the soil would dry out too quickly, so I made plastic pocket liners from inflatable plastic packing material - the kind that looks like pillows. They’re slightly wider than the plant pocket, but I just folded the edges in, and cut the top off to be even with the knit pocket. The plastic liner should also help keep the pocket from stretching out of shape. I punched a few holes in the bottom of the plastic for drainage.

I only did a tie at the top, because I didn’t have access to the back part of the fence to attach a bottom tie - I just hung the pocket over one of the fence boards. It may be a problem in the wind, though.

For the second try, I just put little crochet-chain loops on each corner, and used cup hooks to attach them to the fence, as suggested by my brilliant friend Carrie. The plastic inserts are especially great here, because I can leave the knitted pocket on the fence, and pull out the plastic liner as needed to plant/replant/adjust the plants.