Blooming in July: Basswood

American basswood tree at Queens University

Common Name: White basswood
Botanical Name: Tilia heterophylla
Plant Type: Deciduous tree
Typical Bloom Period: June-July
Nectar Usefulness: high
Pollen Usefulness: medium

A great tree stands at the Harris Welcome Center of Queens University, facing Radcliffe Avenue at Queens Road. It is 67.5 inches in diameter at chest height and its canopy spread is 99 feet. It is an American basswood, Tilia americana.

Most of the Tilia trees growing in Mecklenburg County are instead Tilia heterophylla, white basswood; their leaves are light colored and downy on the undersides. They are sometimes called “bee tree,” because while they bloom for a relatively short time, they shed pollen densely and offer nectar profusely so that they attract an abundance of attention from our bees.

White basswood trees bloom for about two weeks, displaying clusters of 4 to 40 flowers with prominent bracts. The flowers are “perfect”— they have both female and male parts on the same flower, but are generally not self-compatible and need pollinators to initiate fruiting. Most flowers open in the mid-afternoon or evening. Sepals, petals and stamens gradually fold back, and the anthers shed pollen. Tilia are protandrous: the anthers mature first and shed pollen before the stigmas mature to receive it, about 24 hours after the flower first opens. On the second day that a tree blooms, 90% of flowers will have nectar present, sometimes in droplets big enough to see and taste. Peak sugar concentration in the nectar is 28%, and it is a favorite of both diurnal and nocturnal pollinators.

Basswood trees and their European relatives, the lindens and limes (Tilia x europaea, spp.), are a favorite of scientists as well as bees. The pollen shed is so dense that electronic air quality monitors can be used to gather precise data about bloom time. Several published studies have monitored the timing of Tilia pollen shed over time and location. Trees bloom as much as two weeks earlier with just a few degrees increase in annual temperature. They also bloom earlier if they are surrounded by impervious surfaces, such as in a densely paved city.

A visit to the Queens University basswood at the time of this writing will find that it has already bloomed, and the tiny nuts are just beginning to take shape. This particular tree has been featured in a recent book by Margaret Barker Booth, Treasure in the City, and the Queen’s Crown project, www.queenscrown.org, and is certainly worthy of recognition.

Honey from Tilia species is light yellow with a sharp flavor. Pollen pellets are yellow to light orange.

Most of the information in this article came from these studies:

The Pollination Biology of Tilia 
G. J. Anderson. American Journal of Botany Vol. 63, No. 9 (Oct., 1976), pp. 1203-1212. Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2441737
 Flowering phenology of selected linden (Tilia L.) Taxa in relation to pollen seasons.
Agnieszka Dąbrowska, Krystyna Piotrowska-Weryszko, Elżbieta Weryszko-Chmielewska, Ryszard Sawick. Journal of Apicultural Science Vol. 60 no. 2 2016
https://www.degruyter.com/downloadpdf/j/jas.2016.60.issue-2/jas-2016-0030/jas-2016-0030.pdf