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Saturday - August 30, 2014

From: Livonia, MI
Region: Midwest
Topic: General Botany
Title: Is a height of 5 to 8 feet forOenothera biennis (Common evening primrose) normal?
Answered by: Nan Hampton

QUESTION:

I have identified a version of Evening Primrose Oenothera biennia L. In my yard, Livonia Michigan. These plants range in height from 5-8+ feet. Is this typical? The references I find indicates 3-5 feet.

ANSWER:

Our page for Oenothera biennis (Common evening-primrose) says 2-6 feet. The USDA Plants Database Plant Guide gives the maximum height as 25 dm=8.2 feet. Illinois Wildflowers says it grows to 7 feet tall. Ohio Perennial & Biennial Weed Guide says the maximum is 6 feet. University of Wisconsin Robert W. Freckmann Herbarium site says 2-6 feet and so does Minnesota Wildflowers. There are, then, several estimates for height, but I don't think yours are too far out of the ordinary.

There are several factors that can affect the maximum height of any plant. One of these mentioned in the article, Seed weight effects on growth and reproduction in Oenothera biennis L. (by Katherine L. Gross and Marleen L. Kromer in the Bulletin of the Torrey Botanical Club, Vol. 113, no. 3, pp. 252-258) is that the soil type affects the height of the plant. In a soil-sand mix the average height was 88.9 ±15.0 cm (appx. 3 feet 5 inches) and in greenhouse soil (without sand) it was 123.5 ± 4.0 cm. (appx. 4 feet 2 inches). The soil-sand mix was 50:50 regular greenhouse soil:sand. (Incidentally, the seed weight was not shown to have an effect on maximum growth.) Many species that prefer growing in full sunlight react to heavy shade by becoming tall and, often, spindly. Also, there could possibly be genetic variation that would account for the excess height of your evening primroses.