Monday, July 4, 2011

Pasture Thistle Seed

Pasture Thistle that was blooming just a few weeks ago is now ripening seed. The once bright flowers have turned to brown seed heads.

Masses of white fluff dot the landscape, making it easy to find the plants. Pasture Thistles have been quite abundant this year, so I hope there’s a large seed crop.

Most of the plants appear to be releasing seed. There’s been plenty of wind, so it seems odd that the seeds haven’t taken off and moved away from the plant. This pattern of seed drop just doesn’t seem right for a thistle.

A close examination of the pappus shows the seeds to be either absent or undeveloped. I believe birds are feeding on the seeds and dropping the pappus to the side of the flower. This would explain the absence of mature seed. The abundance of seed heads would make them more of a target for seed eating species. As a rule, the numbers of species utilizing a food source increase as the food item increases.

Even when left undisturbed, the seeds seldom move very far from the plant. The pappus itself is heavy and doesn’t have the buoyancy of other thistle species. I imagine this would limit the plant’s ability to colonize distant locations and would help explain why this species is considered uncommon in many areas.

I found these seeds on a plant that was showing minimal bird predation. They look like they need a little more time to reach maturity. Birds don’t mind immature seed. I just hope they leave enough seeds to keep the thistle population from declining.

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