Never fear – it’s easy to add the selvage. In fact, you can have a double selvage that exactly matches the picot cast on, and a single one that makes picots, but doesn’t match. (Because it’s fun to make choices? Why yes, telling people what to do in a pattern instead of offering them choices does feel weird. I even have trouble giving directions instead of options when someone is driving and I am the one who knows how to get there.)
Double picot selvage:
Allow 4 extra stitches.
Row 1: Yo, sl1, k1, psso, work to 2 sts from end. Yo, sl1, k1, psso, turn.
Repeat Row 1 ( and do whatever you are doing in the middle on that row.)
Single Picot selvage:
Row 1: Yo, sl1, k1, psso, work to 2 sts from end. Sl1, k1, psso, turn.
Repeat Row 1 (only do whatever you are doing in the middle on that row.)
And one last note on corners:
If you started with Mary Thomas’ Picot Cast on and plan to continue up the sides with a double selvage, then do the smiling set up with the provisionally cast on stitches slipped onto the initial end and the last few stitches left on the needle as discussed in the cast on post.
Unlike an i-cord corner, you do not need extra repeats to turn, your edges will probably flair (which will be taken care of later if you use the picots to join motifs. Blocking the living daylights out of your piece if also an option.) You do not need extra turns to then use the nearly matching bind off, and you may want to skip a repeat to graft the final few stitches in the final corner.
So, would a picot join after blocking make you more likely to try a modular knitting project? Do your knitting projects go into the closet to die when it comes to seaming up or do you find mattress stitch soothing and appreciate making a large project in small pieces that are bus possible? Have you worked out a join as you go method for knitting?