How do I get smooth color blends in thread painting? My petals look streaky

On the blending, there's no threading trick. Nobody's splitting strands mid-row to get that melted look. What's actually happening is needle blending, meaning people load two colors on the same needle at once. Two strands of the red plus one of the yellow in a single needle, worked in long and short, then flip the ratio in the next row to one red and two yellow. The colors mix inside each individual stitch, which is why it reads as a gradient instead of a band. You can also run a plain row of a midtone, like an orange or an olive, right thru the transition zone if you want extra insurance. The choppy-band problem usually isn't the strands, it's the stitch geometry. If your rows are all the same height and the split edges line up, you get stripes no matter what. Make your stitch lengths genuinely uneven and stagger them row to row, and push each row well into the previous color, a third or more of the stitch length, not just kissing the edge. The split edge of the long and short should sit down inside the older color so the fibers actually interlock. That alone fixes most striped-looking work.

On order of work, I'm afraid background first is backwards for this kind of piece. Big fill areas pull the fabric in and everything stitched on top sits slightly proud, plus your background tension will fight you the whole way. On the next mashup, do the plant first, then fill around it. Before you rip anything out though, don't. Rehoop it drum-tight, and consider a piece of lightweight stabilizer or a second fabric layer behind the hoop if the ground feels at all loose. Then stitch the figure with a deliberately relaxed tension, your needle barely snug, not taut. Cut your threads short between areas instead of traveling them across the back of all that fill, because dragged threads are what pull neighboring stitches down and cause the pucker you were worried about. Three evenings of background is salvageable if you're gentle going forward.

I mostly do cross stitch so needle blending is new to me, but I tried the two colors on one needle thing last year on a sunset piece and the strands kept twisting around each other by the third or fourth stitch. Does that happen to anyone else, or am I pulling too tight? I ended up dropping my needle and letting the floss untwist every few stitches which felt slow but worked okay. Also following this because I have the same stripe problem on a gradient sky I'm attempting. The uneven stitch lengths tip makes sense of why mine looks like blinds, I was keeping every row neat and identical which is apparently exactly wrong.

Can confirm the needle hang trick from the cross stitch reply works for regular surface work too. I did a big leaf gradient last fall with two greens loaded togather and the twist was driving me nuts til I got in the habit of just letting the needle dangle every few stitches while it spun itself out. Also a tiny drop of needle conditioner on the strands cuts the friction so they sit side by side instead of wrapping. Seconding the point about uneven stitch lengths as well, mine finally stopped looking striped once I stopped measuring every stitch to the same height.

One thing I'd add before you commit to the blend is to strip your strands first, meaning seprate each ply and then recombine them, because floss straight off the skein keeps a twist that makes some stitches catch the light differently than thier neighbors and that uneven sheen reads as banding even when the color values are actually fine. I fought this on a peony for ages before I realized the shading was correct and it was the surface luster lying to me. Also check the piece under a raking lamp at a low angle rather than overhead, since flat overhead light hides transitions that become obvious the moment the light moves.