****Warning - picture-heavy post!*****
As promised, when
Examplar IV came through the rotation again, I took detailed pictures as the spiral trellis stitch was worked.

I mentioned in the last post that my first spiral trellis bloom was a bit flat. Here's a close-up of the flower from above.

And from the side.
On to the second flower. I had previously outlined the flower in double-running stitch. I specifically
did not use backstitch because it has more thread on the back side, and therefore has more slack. This is the foundation row for the spiral trellis, and a loose outline would pull into the center and might leave gaps around the flower.

Here is the basic trellis stitch. Bring the thread up a the corner. Slide the needle under the next outline stitch. The needle point should be outside of the circle. Wrap the thread over and under the point of the needle, then slowly pull the needle through.

As you pull away from the circle, the knot will tighten up most of the way. Finish the knot by pulling the thread across the circle.

Slide under the next outline stitch to work the next knot. I stitch the spiral trellis counter-clockwise around the circle, but it can be worked in either direction. If working clockwise, the thread wrap will look different. Just be sure to bring the thread over and under the point of the needle.

Here are two completed knots.

After the first row is worked all the way around, start working into the previous row of stitches. The first stitch of the second row is the trickiest, because you'll insert the needle under the working thread
before the first knot you worked. It's the loop between where the thread came up in the corner and the first knot. If you don't get it exactly in the right place, nobody will really know, though!

Work the rest of the stitches into the loops between the knots of the previous row. See the arrow? It's pointing to the loop between the first and second knots of the first row. That's where the next trellis stitch goes. This is why this is
spiral trellis stitch. It spirals in to the center.

Here's my flower after three rows of spiral trellis. At some point, you might think you have have enough of this color and want to move to the next shade.

Bring your thread to the back close to the last stitch, but don't pull it too tightly. Your spiral trellis is starting to stand up from the fabric, and pulling this tightly will create a dent in your flower! I secured my thread with an
L-stitch so it wouldn't pull any more, then brought the needle through to the front several inches away. After the flower is done, run the thread ends through the outline or neighboring stitches on the back.

With the next color in the needle, start your thread with an away waste knot (to be worked into the outline later) and an L-stitch. Bring the thread up in the middle of the flower close to the next stitch to be made. Notice that I brought it up slightly before the point where the other thread ended. On the next row, I worked a stitch into the little gap between these two threads so it looked seamless.

So far, you're still working the same number of knots on each row, and your spiral trellis is getting taller. If you do this long enough, you can get some interesting effects. You could make a chimney, standing straight up from your fabric. We don't want a flower to be too tall, though, so we need to start
decreasing stitches. To do this, simply skip a stitch. See where the arrow is pointing? That's where the next stitch would be if I didn't start decreasing. Instead, I skipped that stitch, and worked into the next loop.
Judging when to decrease is one of the trickier parts of spiral trellis. The first time I tried this stitch, I ended up with a tall bump that looked a bit like a beehive. (That might be your objective. You could even stuff it! I don't know if this stitch was ever used in stumpwork, but why not?) That's why my first flower on this sampler came out flat. I decreased too quickly that time!

Keep working the stitch into the center, decreasing and changing colors as needed. You'll find you need to decrease more frequently as the circles get smaller. When you reach the center and can't fit in another stitch, stick the needle straight down into the center of the flower and through the fabric. Again, unless you want a dent in the middle of your flower, don't pull it too tight! You can then end off all of your threads.

Here's a couple of comparison shots of my two flowers. Both are fine in their own way, but I think I like the puffy one better. In her original sampler, Catherine Theron made hers much more puffy than mine, then squashed them flat. None of these is wrong. They're just different! Isn't it wonderful that we have so many options, all with the same stitch?

A side-on view shows the height difference in my two flowers. I haven't decided yet if I'll take the first one out. We'll see how it looks when the others are puffy. Maybe I'll make them all be slightly different, so the difference between these two isn't that strange.
If you try spiral trellis, please let me know how it worked for you! Also, please feel free to comment with corrections. I'd like to make sure these instructions are as helpful and as clear as possible!