Rhubarb

Background: 

Rhubarb is a plant used in several different ways in cocktails and spirits. Rhubarb stalks are often used in food items including strawberry rhubarb pie, syrups, and some rhubarb-flavored gins, while rhubarb root is frequently used in bitter liqueurs including Zucca, Sfumato, and Aperol. Rhubarb leaves should not be consumed. 

Useable Parts of Rhubarb Plants:

There are two different types of rhubarb used in food and beverages; "garden rhubarb" that has edible stalks often used in foods, and Chinese/Turkish rhubarb that has roots used to flavor bitter liqueurs.

The United States FDA clarifies that the roots of garden rhubarb may only be used in alcoholic beverages, while Chinese rhubarb may be used as a flavoring generally. 

The United States FDA allows for certain types of rhubarb root as a flavoring [link]:

Rhubarb, garden root (Rheum rhaponticum L) - FL/ADJ, REG, GMP, In alc bevs only - 172.510

Rhubarb root (Rheum officinale Baill., R. palmatum L., or other spp. (excepting R. rhaponticum L.) or hybrids of Rheum grown in China) - FL/ADJ, REG, GMP, In conjunction w/flavors - 172.510 

Garden rhubarb: Rheum rhaponticum (and rheum rhabarbarum, though this doesn't appear to be mentioned by the FDA) is the type of rhubarb used in foods and its stalks are available fresh or frozen in grocery stores and produce markets in the United States and other countries.

• Its leaves contain high levels of oxalic acid and should not be consumed.
• The stalks of this plant have low levels of oxalic acid and are used in foods.
• The roots of this species may be used in alcoholic beverages only, not in food. 

Chinese/Turkish rhubarb: Rheum (palmatum, officinale, and hybrids)  are both used in traditional Chinese medicine and in digestive liqueurs. This rhubarb is typically only available for purchase in dried form. Chinese rhubarb may be used as a flavoring in foods. 

Health Concerns:

Rhubarb leaves contain high levels of oxalic acid and should not be consumed.

Rhubarb has been used as a laxative in traditional medicine; avoid over consumption. 

Chinese rhubarb is often called da huang in traditional Chinese medicine. 

Using Rhubarb: 

As described above, rhubarb stalks can be used as a food item. Rhubarb roots can be used as a flavoring item. Roots of the garden rhubarb varieties may be used in alcoholic beverages only (and thus would be subject to TTB approval).