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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Wednesday - September 17, 2014

From: Magnolia, TX
Region: Southwest
Topic: Problem Plants, Herbs/Forbs
Title: Problem with Pterocaulon pycnostachyum
Answered by: Nan Hampton

QUESTION:

I have a solitary Pterocaulon pycnostachyum in my wildscape; it invited itself. Some years it does not flower at all (or the bloom is eaten before I see it), and its stalks (usually 4) are not strong enough to stay upright. Our sand is barely acidic (we're at the westernmost tip of the Pineywoods) - could it need a more acidic mulch, or ought I to accept that it doesn't belong here?

ANSWER:

If you really do have Pterocaulon pycnostachyum (Blackroot), you are right—it doesn't really belong there.  The USDA Plants Database distribution map for P. pycnostachym shows that Mississipi is the farthest west that this plant is found.  Here are photos from Southeastern Flora.  According to the North Carolina Native Plant Society, it prefers shade and part sun and grows in sandhills, dry pinelands, and pine flatwoods.

However, the USDA Plants Database distribution map shows Pterocaulon virgatum (Wand blackroot) occurring in Magnolia County.  (See the instructions above the map on how to zoom in and move around on the map.)  I suspect that is what you have growing in your wildscape. Here are photos of P. virgatum from FloridaNature.org.

The two species look very similar except for their size.  Here is the description from eFloras for P. pycnostachym and here is the description from eFloras for P. virgatum.   eFloras says that P. pycnostachym is 2 - 8 dm (~8 to 31 inches) and that P. virgatum grows 5-15 dm (~16 to 60 inches).  Other sources (GardenStew and Dave's Garden) say the maximum height is 18 to 24 inches for P. virgatum

There isn't much information that I could find about the growth requirements for P. virgatum.  Our species page has none; Dave's Garden says it grows in sun and partial shade with a wide range of soil pH levels; and GardenStew says full sun and sandy/light soils.

You don't say what the light situation is where you plant is growing.  If it is growing in full shade, this may be one reason it isn't blooming.  Even plants that will bloom in partial shade bloom very reluctantly in full shade and usually bloom most prolifically in full sun.  Additionally, plants that grow in full shade often become tall and spindly trying to reach more light for photosynthesis.  This can also make them weaker and unable to stay upright.  You also don't say whether you are fertilizing the area where this plant is growing.  Fertilizer high in nitrogen can also suppress blooming in a plant.

 

From the Image Gallery


Blackroot
Pterocaulon pycnostachyum

Wand blackroot
Pterocaulon virgatum

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