It is always a good idea to trace back the sources for such claims. The Wikipedia article has this in the footnote:
Schenk, Gustav Das Buch der Gifte translated by Michael Bullock as The Book of Poisons pub. Weidenfeld and Nicolson 1956 page 28, quoting Hovorka, Oskar von and Kronfeld, Adolf : Vergleichende Volksmedizin Zweiter Band. Eine Darstellung volksmedizinische Sitten und Gebräuche, Anschauungen und Heilfaktoren des Aberglaubens und der Zaubermedizin, 2 vols., pub. Strecker und Schröder Stuttgart 1908-9
The second book, Oskar Hovorka und Adolf Kronfeld, Vergleichende Volksmedizin : eine Darstellung volksmedizinischer Sitten und Gebräuche, Anschauungen und Heilfaktoren, des Aberglaubens und der Zaubermedizin, two volumes is available online vol. 1 vol. 2. In volume 1, page 421f, the Tollkirsche (belladonna) is treated and some examples of traditional uses are listed. The one that was cited in in the WP article has another source:
Dan, Demeter: Exarch und Pfarrer.
Volksglauben der Rumänen in der Bukowina. Zeitschr. f. österr. Volkskunde II
250; III 20, 116, 180, 370; IV 213 und VII 255.
Hanns Bächtold-Stäubli, Eduard Hoffmann-Krayer, Handwörterbuch des Deutschen Aberglaubens, de Gruyter 1927–1942 vol. 8, p. 1101f tells the same story and gives the same source.
The periodical Zeitschrift für österreichische Volkskunde fortunately also has scans of its first few volumes. As it turns out, this Demeter (Dimitrie) Dan, a Romanian Orthodox parish priest in the town of Straja, had collected more than six hundred small tidbits of Volksglaube (popular belief) and published them in a seven-part series under the periodical's general heading Kleine Mittheilungen (small communications) between 1897 and 1901.
To give an impression what sort of things he wrote down, the following is not only the paragraph about the belladonna root (no. 174), but some of the entries immediately before and after. It can be found in the second installment, issue 4 of 1897, page 117:
- Kinderwindel dürfen nicht als Fußfetzen benützt werden, ansonsten würden die Kinder aus dem Munde übel riechen.
- Die Mädchen schmücken sich, um zu gefallen, mit Blumen, stecken dieselben aber nie unmittelbar nach dem Pflücken in den Gürtel oder Busen, sondern ziehen sie zuerst durch's Feuer oder die Flamme eines Lichtes, damit sie vor zugesandtem Abscheu gefeit werden.
- Wenn sich ein Hund in einem Blumengarten eines Hauses, wo sich ein erwachsenes Mädchen befindet, verunreinigt, so glaubt man, dass jenem Mädchen der Abscheu zugeschickt wurde.
- Wenn ein Mädchen den Burschen gefallen und beim Tanze das erste sein will, da muss es an einem Sonntage im Fasching mit seiner Mutter, in Sonntagskleidern angethan, in's Feld gehen und im Kurzsack Brot, Salz und Brantwein mitnehmen und eine Tollkirschwurzel ausgraben und dann an ihre Stelle die mitgebrachten Sachen zurücklassen. Auf dem Heimwege muss es jene Wurzel auf dem Haupte tragen und muss beim Hin- und Zurückgehen jeden Streit und Zank vermeiden. Sollte es befragt werden, was es denn nach Hause trage, so darf es nicht die Wahrheit sagen, denn sonst würde dieses Mittel nicht helfen.
- Man glaubt, dass es nicht gut ist während der Nachtzeit zu nähen oder zu schneiden, denn sonst würde man leicht mit einem gebrauchten Werkzeuge verwunden.
- Ein Vielesser wird für einen Egoisten gehalten.
- Man glaubt, dass aus dem am Donnerstage unter die Gluckhenne gelegten Eiern nur Hähne herauskommen werden.
- Children's nappies must not be used as footcloths, otherwise the children would smell bad from their mouths.
- Girls adorn themselves with flowers to please, but never put them in their belts or bosoms immediately after picking them, but first pass them through the fire or the flame of a light, so that they are protected from antipathy sent to them.
- If a dog defile itself in a flower garden of a house where there is an adult girl, it is believed that antipathy has been sent to that girl.
- If a girl wants to please the boys and be first at the dance, she must go into the field with her mother on a Sunday in Carnival, dressed in Sunday clothes, and take bread, salt, and brandy with her in a short sack, and dig up a belladonna root, and then leave the things she has brought with her in its place. On the way home, she must carry the root on her head and avoid any quarrelling and bickering on the way there and back. If she is asked what she is carrying home, she must not tell the truth, otherwise this remedy would not help.
- It is believed that it is not good to sew or cut during the night, otherwise one would easily be wounded with a used tool.
- An overeater is thought to be an egoist.
- It is believed that only roosters will come out of the eggs laid under the broody hen on Thursday.
(According to kioleanu's comment, the Romanian word for "Shrovetide" is Lăsata Secului which quite literally translates to "the dawning of the dryness". Carnaval is used to refer the Shrovtide in other countries, where people dress and party before lent, but Romania has no such tradition. The German word in the Bavarian group of dialects is Fasching, which is the one used by Dan for his Austrian readers.)
Since these pieces of popular wisdom were marked as "collected by Demeter Dan" (presumably from oral interviews in the late 19th century), more details are unlikely to be found. On the other hand, I have seen a few other references to Carnival in the series, so a thorough reading could turn up something that might serve as context.
One thing seems to be obvious to me: the dances the girl wants to shine at would be those of the Carnival season, as afterwards in Lent there will be none until Easter.
