We are continuing to profile noxious weeds in order that we all get to know them. Although not the most fun aspect of gardening, it is important that we help to control their spread so as to avoid impacting our native plant and animal populations, and in some cases, helping to reduce the spread of weeds into our own gardens. An added benefit of learning to recognize weeds is that any plant that appears in our garden that we do not recognize to be a weed, may be left to grow. Wonderful wildflowers and other desirables can thus windfall their way into our gardens.
One of the most common noxious weeds in our area is Canada Thistle, AKA Creeping Thistle, or Cursed Thistle, Cirsium arvense. Originally native to southeastern Eurasia, it was introduced as a contaminate in crop seed and has naturalized extensively in the US. Although not the tallest member of the thistle tribe, only 1’-4’ high, it may be the most pernicious, spreading aggressively not only by seed but also by deep and extensive horizontal roots. Unfortunately it is also a perennial, so mowing or removing flowers before seeding does not defeat this weed; neither does plowing or rototilling as chopping up the roots only spreads it. The flowers are pinkish purple with small heads ½”-3/4” in diameter which bloom July/August. The leaves are spine-tipped and lobed. I am having success removing Canada Thistle from a garden using a knife-weeder, but it has taken three years of inconsistent weeding.
The other thistle on the list of the top 10 prioritized weed species for Colorado is the Musk Thistle, or Nodding Thistle, Carduus nutens. This weed has a significant presence, growing 2’-6’ high and is widely branched. The leaves are deeply lobed and very spiny with a light green midrib; young seedlings have wavy margins. The flowers bloom in May/June and are large 1 ½”-2 ½” in diameter, deep rose, violet or purple and usually bend over (nodding). Even though they are biennial, they form dense colonies. Control is by stopping seed production, mowing or cutting the plant off below the crown.
Bouncingbet or Soapwort, Saponaria officinalis was introduced from Europe as a garden plant and is a prohibited ornamental in Colorado. Naturally the prohibition is against selling it, for prohibiting this bouncing herb would be like attempting to prohibit poverty in the third world. This common soapwort is a perennial, growing 2’-3’ on smooth, branching stems which sucker along as it spreads. The flowers are phlox-like, five-petalled and in clusters at the ends of the stems; they are pink, blush or white, blooming in summer. In pioneer days the stems and roots were crushed in water to make a soap (soapwort) and can still be used to make a mild soap for delicate fabrics. Control is by preventing seeding and by clean cultivation. It is quite drought tolerant and can be seen in large colonies when it blooms.