Seaming garter stitch pieces together is very simple once you learn how and the seam is practically invisible.
All you need to do is follow the flow of the bumps on each garter stitch ridge. And using the mattress stitch once again you will create a nice invisible seam.
Have a look at this Garter stitch photo
See how each ridge has a top bump and a bottom bump?
All you're going to do is seam into a top bump and then on the other piece seam into the bottom bump. Actually it's more a weaving technique than mattress stitch but the thing is it looks great when you're done.
And it keeps it all even and creates an invisible seam.
Thread your yarn needle with the yarn end if it's long enough to seam with. If not get a fresh piece and just cut about 10 inches
First you're going to join both garter stitch pieces. Push you yarn needle through the last stitch on one of your finished pieces and then do the same on the other piece.
To start with I decided that on the left piece I'd go into the bottom bumps so push your yarn needle through the bottom bump and bring it right through the loop.
Move over to the other piece and push the yarn needle through the top bump all the way through see left photo. Move over to the other piece and push yarn needle through the bottom bump, see right photo
Keep working each side the same way, going into a bottom bump, then going into a top bump.
Oh boy here is the magic moment. After about an inch to an inch and a half gently but firmly pull the yarn to bring both pieces to together. Play with it a bit too so that the stitches line up nicely.
Just keep seaming garter stitches all the way up to the top of your piece of fabric. Pull it all together like in the photo.
Here's how I bring it all together at the top. Push yarn needle through the loop beside the loop the yarn is coming out of. (Hmm does that make sense?) Check out the photos
There you go. Push yarn needle through the loop to secure it.
Then just snug up the yarn a bit and it's all ready to weave in.
And this is how it all looks when you're done. The left photo is the right side and the right photo is the wrong side. You can see the orange a bit but when you use the same yarn it won't show. And you can play with the stitches along the seam a bit too so that they line up better.
This is the seaming technique I use the most for seaming garter stitch.
There are some who believe that when you seam the edge stitches it makes for a very flimsy edge. In all honesty? I've never had a problem. My knitting never fell apart and it works well for me.
However there is another way for seaming garter stitch if you are worried and want a super duper firmer edge.
Actually there is only one small change and it's really simple too.
If you want a firmer edge to your knitted seam you can try this seaming technique.
Join your two pieces just like I showed you above. Then push yarn needle through the top bump on the left piece at the edge. (You can choose whichever bump you want to go into at the start. Just remember to go into the corresponding bump on the other side (bottom and top or top and bottom))
Here's where seaming garter stitch this way changes ever so slightly. Move over to the other piece of fabric and skip the first bump. Push you yarn needle through the next bump which is the bottom bump all the way through.
Keep working the seam in this way. On one piece (left photo) push yarn needle through the edge bump and on the other piece skip one stitch and push yarn needle through next stitch. They should both correspond with each other. In the photos the left one I go into the top bump and the right one I go into the bottom bump.
Once again, after about an inch gently but firmly pull the yarn to bring pieces together and then continue.
And you're all done. The left photo is the right side and the right photo is the wrong side. There is a small seam on it because you didn't work that one edge stitch but it's OK. No one will see it and it gives you a much firmer edge to your garter stitch seam.
Here is a great video that shows you the same seaming technique using the edge stitches just like the first set of photos I showed you here.
Have a look..
I hope this helps. Happy Knitting!