This picture might not look like much to you, but it's been the source of serious pleasure to me! It's a picture of cotton interlock finished with rib-knit binding. And not a pucker, stretch or ripple to be seen.
I posted my plea for help on the Ottobre Yahoo! group. And as well as some very sweet comments on the puckersome hoody, I got some excellent advice from Margaret (aka Once and Future Stash Empress). (Side note: If you want to improve your sewing, this Yahoo group is awesome. The members are uniformly polite, friendly and knowledgable. They will pour their sewing expertise over you - you just can't help but get better! And the Ottobre magazines have some of the best sewing patterns around - they're definitely worth the extra step of tracing them.)
But to return to the rippled seams: if you have sufferered from horizontal seam stretching on knits in the past and want to know how to de-pucker, Margaret recommends that you:-
1. reduce your presser foot pressure;
2. increase your stitch length;
3. (for overlockers) reduce the differential feed.
She did so in a beautifully lucid post, which explained exactly why these three steps would work. And they did! I had a happy half-hour this evening playing around with some interlock scraps and making little samples, to which I have attached notes setting out their stitch length, width, presser foot pressure and (for the serged sample) differential feed.
So all that remains is for me to retrieve the hoody and re-do the ribbing. Oh, and come up with another name for the boys' clothes collection. It turns out that 'Les Pets' on the label will set French speakers guffawing because "pets" means - well, I'd rather not say in polite company but here's a clue: it's an anagram of RAFTS. Go figure.
Addendum: Nicole from Dots & Stripes has left a comment on the Hoody post, advising that she reduces the presser foot pressure when she does horizontal seams on knits. And if you check out her blog, you can see that it really does work!