Wednesday, August 29, 2012

Short Field Thistle

This is the time of year to enjoy the lovely blooms of the tall native thistles.  These gangly giants can reach a towering 9 feet tall and produce dozens of separate flower heads.  Each bloom is like a starburst fireworks display.  The pictured creation was produced by the common Field Thistle, Cirsium discolor.

Like many other plants of the prairie, Field Thistle failed to live up to its expectations this year.  At 24 inches tall, this plant barely makes it above the surrounding grass.  The effects of an early season drought continue to be demonstrated by the short stature of normally tall and robust plants.

Field Thistle is usually a short lived perennial plant in this area, so it began its growth by using root stored energy from the previous growing season.  As the leaves developed, the plant was no longer dependent on stored energy and began producing its energy through photosynthesis.  The first priority for this newly produced energy is for root growth.  It’s easy to see the cycle of death and regrowth of the above ground portion of the plant.  What goes unnoticed is a similar cycle with the roots.  As the growing season progresses, the old root mass declines in favor of fresh new growth.  The success of the root replacement process strongly influences that amount of energy available for leaf growth, flowering and seed production.  The plant’s priority is to amass a store of energy sufficient to carry it though the coming winter.  In extremely stressful years, this may mean no flowers at all.

Fewer flower heads means that flower visitors have to crowd in and share the bloom.  Most flowers contain a mixed bag of beetle species.

Many people are quick to tag all tall growing thistles as the non-native Bull Thistle.  A check of the stem will quickly separate the Bull Thistle from the natives.  The stem of the Bull Thistle contains spines, while the native thistles have a spineless stem.  The leaf tips still carry a full arsenal of sharp spines, so it’s best to be cautious around these plants.

Even a handful of flower heads will produce a large quantity of seed.  Seeds are distributed by the wind, but the birds will take most of them before they are fully ripe.  It appears that the fight for seeds will be highly competitive this year.  I hope deep, persistent snow cover doesn’t add to the problems related to a diminished seed crop.

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