Rheum rhabarbarum (Introduced- USA/ Germany)

Rheum rhabarbarum L., Sp. Pl. 372 1753. (Syn: Rhabarbarum verum Garsault; Rheum franzenbachii Münter; Rheum franzenbachii var. mongolicum Münter; Rheum undulalum L.; Rheum undulatum L.; Rheum undulatum var. longifolium C.Y. Cheng & T.C. Kao) as per The Plant List Ver.1.1;
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S. Siberia to N. & Central China: Buryatiya, China North-Central, China South-Central, Chita, Inner Mongolia, Irkutsk, Manchuria, Mongolia; Introduced into: Alaska, Alberta, Colorado, Connecticut, Czechoslovakia, East European Russia, Illinois, Indiana, Iowa, Korea, Labrador, Maine, Manitoba, Massachusetts, Michigan, Minnesota, Mississippi, New Brunswick, New Hampshire, New York, Newfoundland, North Carolina, Nova Scotia, Ohio, Ontario, Pennsylvania, Québec, Rhode I., Saskatchewan, Utah, Vermont, Virginia, Wisconsin, Wyoming, Yukon as per POWO;
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Fruits & Vegetables Week: Rheum rhabarbarm, Rhubarb a fruit that is not a fruit:  Attachments (2). 2 posts by 2 authors.
From California.

Fruits & Vegetables Week: Rheum rhabarbarm, Rhubarb a fruit that is not a fruit from Ritterhude:
Here some fotos from my garden taken in may 2010.
The leaves of this plan are NOT used as vegetable, as they contain poisonous substances, including oxalic acid. Mainly the petioles are used to make jam and in cakes and desserts. Jam of rhubarb with strawberry is very popular. In may-june the rhubarb cake is sold and bought everywhere. After june the amount of oxalic acid in the plant increases and though there are quite a few petioles still coming out if the soil, we don’t consume them anymore. Because of the oxalic acid people with osteoporose are adviced not to eat Rhabarber.
In english wiki it says Rhubarb has been used for medical purposes by the Chinese for thousands of years.
In german Wiki I found, that originally it comes from Himalayan region. So don’t know the correct origin.
My information is a bit different than yours. Perhaps we are using different sources of information. But not using leaves, not after june etc. is practised here in germany and I suppose in other countries too.
In the first foto in the upper right corner the white flowers are of Garlic mustard (Alliaria petiolata)  german: Knoblauchsrauke.

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Please note that Rhubarb contains oxalic acid also in the petioles so it is not recommended for people that have kidney problems. There are now new garden cultivars that are almost oxalic free and taste much sweeter than the old ones. You can use rhubarb during the whole growing season but the spring growth is more vigorous and easier to harvest than the few leaves that grow later in the season.

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I did not communicate for a long time because of lack of time. I am afraid, this will not change in the near future. But I remember you all and your work, especially, now that the gardening season has started.

I think the fotos are from my garden.
As I am not a botanist, I cant “analyse” the Rhabarber I have in the garden.  We here exchange many plants with friends etc. They don’t have any Tag, what the exact name of the Plant is. And most of the people differentiate between edible and non-edible. That applies to mushrooms too. That is why i am not able to answer mushrooms-queries either.
I can just say, this Rhabarber is cultivated in Germany. Though I am not sure, if another variety is also cultivated here. Sorry.

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The right photograph at least is clearly Rheum ×rhabarbarum L. Petioles are commonly sold here in California. These are red and colour and eaten raw as fruit. We used to give this as an example of fruit which is not botanically a fruit in our teaching classes.

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yes we eat here them raw or to make compote with strawberries, to cover the cake.
It is also one of the important RUMTOPF ingrediants. Rumtopf, I think, is typical german.
We start in April / may, until August different fruits are added, in Winter is the time to try it, sitting next to the fire.
http://en.wikipedia.org/wiki/Rumtopf

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Polygonaceae Fortnight: Rheum ×rhabarbarum L. from California-GSMAY62 : 1 post by 1 author. Attachments (2).

Rheum ×rhabarbarum L.
Rhubarb petioles sold in market in California. Often cited as example of fruit which is not botanically a fruit. Eaten often raw

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identification AH1 : 2 posts by 2 authors. Attachments (1).

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Seems a species of Rheum (Polygonaceae).

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Seems to be Rheum rhabarbarum

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References:
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:301818-2

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Flora of China (Rheum rhabarbarum Linnaeus syn: Rheum franzenbachii Münter; R. franzenbachii var. mongolium Münter; R. undulatum Linnaeus; R. undulatum var. longifolium C. Y. Cheng & T. C. Kao.) 

GRIN (Rheum undulatum L. syn: (=) Rheum franzenbachii Münter; (=) Rheum rhabarbarum auct. E. Asian)   

Flora of North America (Rheum rhabarbarum Linnaeus)