Many knitters like seams to be invisible and single crochet, for sure, doesn’t produce an invisible seam. But here’s the thing : invisible seams are not as sturdy as a single crochet seam and a sturdy will help your garment to keeps its shape. We’re talking cardigan, sweaters, T-shirts, vests and tank tops here.( I’m using US crochet terminology here, so for my fellow British knitters, we’re talking “double crochet” (dc)).
Do I seam all my knits with single crochet ? No, I don’t. But I seam with crochet all my knits that I made with inelastic yarn (cotton, bamboo, silk, linen). So far, I have never knitted a garment solely made of silk, linen, hemp or viscose, but all these yarns are inelastic. A sturdy seam helps them keep their shape longer. And an item that keeps its shape longer is an item that you can wear for a longer time. Sustainability comes in seam choices too.
Compared to a fibre like cotton, wool is less dense, with air trapped inside the cell structures of each hair. As Margaret Radcliffe explains : “If you take two pieces of yarn, one cotton and the other wool, that are the same thickness and the same length, with the same number of plies and the same amount of twist, the cotton yarn will be heavier than the wool.” And this has consequences on how the knitted garments will behave through time and as you wash them : “Cotton garments, because they are heavier, tend to gradually stretch in length while worn. If they shrink in the wash, they will usually end up shorter and wider. Wool garments, because the yarn is elastic and less dense, hold their shape better. If they do stretch out of shape, washing restores the fiber’s natural crimp, and they can usually be blocked back to their original shape.”
Now how can a sturdy seam help a garment keep its shape ? A sturdy seam at the shoulders will help carry the weight of your fabric. This is important for all inelastic fibres and it’s especially important if you are plus size as the seam is holding together two pieces of fabric that weight more than if you had a smaller size. So, if you only agree to have a single seam that is done using single crochet, I would suggest seaming your shoulder seam that way. I am myself a size M/L. Maybe if you are a size XS, the fact of not having a sturdy seam at the shoulders isn’t such an issue, I really don’t know. I have never knitted anything for someone in that size.
Now, as mentioned above, inalestic fibres tend to become wider and wider through time. Seaming the side seams with single crochet helps your knit keep its shape. In fact, this seams can do a really good job at that, even for cheap yarns. Thus you get to wear your summer knits for several years. Should you get fed up of it, you could give your knit to a charity and it would be a valuable gift because it would still be a nice-looking and wearable item. I rest my case : sustainability comes in seam choices too.
I am fully aware that using single crochet is NOT the most common way of seaming pieces of garments together. And if a knitwear designer instructs you to do “a crochet seam”, they don’t intend you to do a single crochet seam. What they intend you to do is a slip stitch crochet seam. This is slighlty sturdier than a mattress stitch seam, but less than a single crochet seam. On the right side of the fabric, a single crochet seam and a slip stitch seam look very similar. But on the back side of the fabric, a single crochet seam creates more of a bulk than slip stitch crochet.
References :
Margaret Radcliffe. The Knowledgeable Knitter. Storey Publishing, 2014., p 24. (both quotes above are from the same page).
Video from Tamara Kelly where she demonstrate how to seam a knit using a slip stich seam and a single crochet seam. Note that for both techniques, she is going with her hook under both sides of each stitch of the cast off stitches, in a way that is very similar to how you pick up your stitches when you want to start knitting in another direction. When seaming, I like to use a crochet hook that is one size smaller the size of my needles, but using a hook of the same size works too.
Unsolicited Knitting Advice 