If you were to do a search on this forum you would find that I asked a similar question last year regarding this plant...

My R. palmatum var. tanguticum was planted in the ground in spring of last year and by summer had yellowing foliage that would kill each leaf off one by one. I suspected that it was too dry where I first put it, so moved it (root ball and one whole yellowing leaf, lol) into a half-wine barrel planter behind my shed. It is shady there, and very cool. I put a heck of a lot of compost in the barrel too. The last lone leaf died soon after, so I wasn't all that sure it would come back this year. But it did!! I was so thrilled.

From what I have seen, this plant can be a bit fussy. The main issue is that it doesn't like the heat/a situation in the garden that isn't nice and moist. I noticed that some of the leaves are yellowing again now, though it is putting out new growth at the same time. I'm thinking that is partially the nature of rhubarb, to die back a little toward summer and then flush back out again. That is what my culinary one does when I don't cut it for use anyway, lol.
CMK

Nice plant. I guess it was our extreme heat the did the in. I kept then nice and watered the whole time. I might heave to try the or other varities next year. I love the giant leaves.

Did all your plants die then? I collected a bit of seed from my plant (least I think it is true seed, not quite as firm as I expected...) and could spare a bit if you are interested. Shoot me an email if you are. ;-)
CMK

I have observed that this plant goes dormant very early, especially if stressed by drought or heat.

To the OP, are you sure your plants are dead, and not just dormant. If you haven't thrown the pots out yet, I would gently probe down in the soil and see if see live roots and any small dormant buds (which I seem to recall are sort of pink/purple in color and slightly pointed, similar to a peony dormant bud in appearance).

I think it's quite possible your "dead" plants are just dormant for the season.

Cmk... Thanks.. I will email you. I will try to grow again next year and keep more in the shade.. Maybe bring them in if we get another heat wave like this year. Hopefully if the plant can grow out a bit more without the stress... It could take it the following year.
And about the dormancy... I had read about how they will loose leaves in the height of summer but then come back... So I did check the root stock and they all were mush.