Proceed at your own risk. Sources say that the leaves and roots are edible BEFORE it flowers, and PFAF says that the buds can also be used as a caper substitute. Meanwhile, PFAF says that it is harvested for medicine while it is in flower.

LESSER CELANDINE -  Ranunculus Ficaria

  Flowers (2425723494) cropped.jpg       

FROM: https://pfaf.org/user/Plant.aspx?LatinName=Ranunculus+ficaria (The advice below appears somewhat conflicting.)

CAUTION: All parts of the plant are poisonous. The toxins are unstable and of low toxicity, they are easily destroyed by heat or by drying[19]. The sap can cause irritation to the skin[65]. Do not use internally. Stop using the herb if breathing problems or chest & throat tightness [301].

EDIBLE: leaves & root - Young leaves in spring - raw or cooked as a potherb[2, 74, 105, 183]. The first leaves in spring make an excellent salad[9]. The leaves, stalks and buds can be used like spinach[9], whilst the blanched stems are also eaten[46, 183]. The leaves turn poisonous as the fruit matures[74]. Caution is advised regarding the use of this plant for food, see the notes above on toxicity. Bulbils - cooked and used as a vegetable[9, 105]. The bulbils are formed at the leaf axils and also at the roots[9, 183]. Caution is advised, see the notes above on toxicity. The flower buds make a good substitute for capers[183].

MEDICINAL: Lesser celandine has been used for thousands of years in the treatment of haemorrhoids and ulcers[254]. It is not recommended for internal use because it contains several toxic components[254]. The whole plant, including the roots, is astringent[4, 165, 238]. It is harvested when flowering in March and April and dried for later use[4]. It is widely used as a remedy for piles and is considered almost a specific[4, 238]. An infusion can be taken internally or it can be made into an ointment and used externally[4, 238]. It is also applied externally to perineal damage after childbirth[238]. Some caution is advised because it can cause irritation to sensitive skins[244].