Maiden Grass 'Morning Light' - when is the best time to plant it? WE live in Zone 6b
Now would be a great time to get your Maiden Grass planted. Early spring is generally recommended. Here's an article with more information:
https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass/morning-light-maiden-grass-care.htm
We have 3 plants in our garden. I have just noticed what looks like small but new thick stems of knotweed growing in the middle of all 3 and at the side but close to the existing shrub. Could these simply be new stems as I can’t believe that knotweed has suddenly decided to grow in 3 plants all in a completely different area. I have googled new stem images of miscanthus and they do look very similar to knotweed. Hope you can help.
Do you mean nutsedge or crabgrass rather than knotweed, because miscanthus does not look like knotweed. Miscanthus would be coming in the center and right at the edge of the clump, not usually away from it. But other grassy weeds such as nutsedge or crabgrass might. It is probably your miscanthus returning, so I would let it grow until you know for sure.
https://www.gardeningknowhow.com/plant-problems/weeds/controlling-sedge-weeds.htm
https://www.gardeningknowhow.com/lawn-care/lgen/lawn-weed-identification.htm
Last year they were both doing good but not this year. one only has a few grasses coming up.
Unfortunately, your photos did not come through. I am unable to see the damage.
Since you are at the lower end of where they can survive, I would expect winter damage to be a little more damaging to this grass. With this being said, any maintenance or pruning should be done in spring when you first notice new growth to avoid frost bite to the root zone.
Here is a collection of articles that will help you to care for the grass:
https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass
Does this ornamental grass rhizome? My concern is will it spread uncontrollably? I could plant them in plastic pails with the bottom cut off.
In such a mild area, it would tend to become invasive. I would have care, and I think that your proposed method would help quite a bit.
https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass/growing-maiden-grass.htm
https://www.gardeningknowhow.com/ornamental/foliage/ornamental-grass