Saturday, September 25, 2010

Rosette Gall

When I see a tightly packed rosette of leaves such as these, I think of a ground hugging plant. Many species of plants begin development as basal rosettes before sending up flower stalks. This, however, is not that type of plant.

This rosette forms on top of a stalk and is normally a couple of feet in the air. It’s an interesting plant, but that is certainly an odd stalk topper. The plant is the Canada Goldenrod and the cluster of leaves is a gall formed by the larva of a tiny midge.

The female midge, a type of small fly known as Rhopalomyia solidaginis, lays its egg in a bud at the growing tip of the plant. When the egg hatches, the presence of the larva inhibits stem elongation, so the plant grows no taller. Leaves continue form and develop one atop the other until you have a tightly packed cluster.

The plants still manage to flower despite their lack of height. The midge doesn’t hurt the seed producing potential of these plants even though the total flower production is slightly diminished.

Cut in half, the gall looks much like an artichoke. Without the interference of the midge, each leaf would have been separated by a section of stalk.

The cluster of leaves provides a place of safety for the midge larva to grow and pupate. The leaves will brown and whither over the winter, but the gall will hold together and continue to provide protection to the pupa. The adult midge will emerge next spring.


  1. Hi Steve...That is very interesting...I have seen plants have similar things happen to them but never really seen what caused it!!
    Always interesting stuff in word and photo!!

  2. Great observation, Steve. The rosette in the first photo is actually quite attractive. We all need to be thinking about how we will overwinter, don't we? ;-)

  3. I have seen Gall on Aspen around here. They are small and look like warts.

  4. Hi grammie g. I’ve seen this same type of effect on some other species besides goldenrod, but never cut those open to see if anything was inside. I’ll have to do that next time I find one.

    Wilma – I did pick the cutest rosette to use for a close-up. I just think rosettes are the neatest plant form. I’d like to have a garden bed full of resetted goldenrod.

    Hi Rosey. I don’t have Aspen here, but I’ve seen warty galls on maple, elm and hackberry. There are all kinds of different galls here, all caused by different things.

  5. Cool Steve! I knew they were galls, but had not split them open to find the critter. Great post!