I have been thinking of adding this for a while but it is hard to describe and a chart would be easier to see but for what it is worth I will add the following.
Now I am not claiming that this is the best possible way but I found it quite effective.
To fill in the curves you make parallel rows of normal three treble granny groups.
I am not going to say which side is the right or the wrong side as a blanket like this doesn’t have a right and wrong side until you add a border that is one sided. I notice with my two granny ripple blankets I have chosen different sides as the ‘right’ side before making the border and I am happy with both decisions.
For the Spectrum blanket it looks best (in my opinion) to make the first row the ‘right’ side. This gives a slight ridge in the middle of each colour stripe, so you get one ridge for the two row stripes and two ridges for the four row stripes. You can see this in the pictures below.
For my ‘random’ stripey blanket I chose to make the other side the ‘right’ side as you then lose the sense of the ridges altogether.
Top (final row) end
Start by turning the blanket as if you were working in the other direction to the last row, you are going to begin by making two 3tr groups into the central gap at the bottom of the curve.
First make a slip stitch into the gap to the right of centre to fix yarn then work (3trs, 1ch, 3tr) into the central gap. Then slip stitch into gap to left and turn work. (2 groups)
Second row – 3ch, slip stitch into gap to right, 2trs into gap where 3ch began gives first granny group. Then work 3trs into gap, then 3 more trs into gap where you made first slip stitch. Then slip stitch into gap to left and turn work. (3 groups)
Next three rows – 3ch, slip stitch into gap to right, 2trs into gap where 3ch began gives first granny group. Then work 3trs into each gap, including gap where you made first slip stitch on previous row. Then slip stitch into gap to left and turn work. (4 groups) , (5 groups), (6 groups).
The last gaps you are working into at this end of the blanket should be the 1ch between two lots of 3trs at the top of the curve.
When this is worked into every lower curve you should have two groups of 3trs from the original last row between six new treble groups.
Bottom (starting) end
Obviously you turn the blanket to have the same side towards you as you had at the other end and the curves at the top.
Except that you work your first two groups of 3trs, one each into the two central gaps, the procedure is as described above.
Here all the gaps will be the same as they are all evenly spaced and will be wider than at the other end as they comprise two chains.
This end you also have the corner half curves.
Maybe I should leave this as an exercise ! 😉
Filling in the half curve on the right hand side.
Start in the end gap and work 3ch then 2tr into the gap, then the slip stitch into the next gap and turn as before.
Second row – 3ch, slip stitch into gap to right, 2trs into gap where 3ch began gives first granny group. Then work tr into 3rd ch at start.
Third row – 3ch, slip stitch into gap to right, 2trs into gap where 3ch began gives first granny group. Then 3trs into next gap. Then the slip stitch and turn as before.
The other rows are worked similarly to two and three, alternately, so you will have, 1 group, 1 group + tr, 2 groups, 2 groups + tr, 3 groups.
Filling in the half curve on the left hand side.
Here you will start with a slip stitch into the gap to the right, then the 3trs into the gap and 3ch to form the start of the next row and turn.
Second row – 3 trs into gap where you began with a slip stitch, slip stitch into gap to the left and turn
Third row – 3ch, slip stitch into gap to right, 2 trs into gap where 3ch began gives first granny group. Then 3trs into next gap and 3ch to form the start of the next row and turn.
The other rows are worked similarly to two and three, alternately, so you will have, 1 group, 3ch +1 group, 2 groups, 3ch +2 groups, 3 groups.
The above forms a basis on which you can work a straight row of stitches. Another row of treble groups into the ends is quite a good idea, together with 4trs into all the obvious gaps down the sides as a basis for an edging but obviously other choices are possible.
With my Spectrum blanket I experimented a bit with using three and four trebles into the gaps at the sides and decided that three was not enough but four was slightly too many. Four was closer and to my mind the logical amount and so as I wanted a firm rather than fluid border, I went down a hook size from 5mm to 4.5mm. I would still go with four even if keeping the same hook size but you may disagree.
I suggest the trebles row, as described, before the edging as it evens things out nicely.
Here are some (I hope) helpful pictures.
I have edited the photographs I took while making my spectrum blanket and just shown some of the stitches as I think this should be clearer.
At the top
The thick arrows represent the granny 3trs. I have shown the ones worked between those of the top row of the fill-in in white. The yellow ones are the ones worked into gaps in the final row of the main blanket. This general pattern will have been repeated all along the top.
5trs in the end stitch are to turn round the corner then I work 4trs into the obvious gaps down the side as shown.
For my spectrum blanket I worked all the ones shown in the Bluebell and then continued down the side with the 4trs in the Silver yarn till I got to the last silver stripe when I went back to Bluebell.
At the bottom
This shows how there are five lots of 4trs in Bluebell for the Spectrum blanket. Even if you don’t change colour there should be one lot in the last double row and four lots of 4trs in the fill in.
Again there are 5trs in the corner stitch and two granny 3tr groups worked between those of the top row of the corner fill-in. There are three worked into the original bottom edge of the blanket but two of these are worked into gaps and one into the chain that the middle 3trs had been worked in on the first row. (As you should be able to see in the picture). With five granny 3tr groups worked between those of the top row of the main fill-in. Then back to the three shown in yellow and so on.
This does give you the same number of stitches along both the top and bottom edges.