Weekly Express-News Article

By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist
Saturday, July 19, 2008

 “Rain and Grassy Weeds”

The rain we have received over the last two weeks is a blessing for the aquifer and our landscapes. One negative thing that will happen is that the weed seeds will sprout.  Sandburs and crabgrass are annual weeds that grow from seeds.  Bermuda grass can sprout from seeds, but it will also spread vegetatively from runners that move across the surface of the ground and rhizomes that run in the soil. 

Some crabgrass and sandbur germination might still be prevented by applying a pre-emergent herbicide like Amaze, XL, Balan or Betasan, but most seeds will germinate too quickly for the pre-emergent to do its job.  It is usually applied in early April and then again in late May. 

Be careful about applying a pre-emergent this late in the season if you expect wildflowers to germinate this autumn.  We have a dismal wildflower bloom this spring, but there are plenty of viable seed in the soil on most sites from previous years that will bloom this fall if the rains continue.  The pre-emergent herbicide will prevent sandburs and wildflowers from germination for at least three months.  

If you have a Bermuda, buffalo or zoysia grass lawn, a MSMA product will kill the young crabgrass and sandbur plants. Apply it according to the instructions on the label.   MSMA also kills St. Augustine grass so it is not an option if the two weeds grow in a St. Augustine lawn. The new sandbur and crabgrass plants are also relatively easy to hand-remove if the area is not too large.

In a vacant lot, Round-up can be spot-sprayed on the thick bladed grassy weeds that emerge over the next few weeks. If the weeds are not killed before they produce seed, there will be a bigger crop next year. In the case of sandburs, if you are conscientious in killing the young plants over the next few weeks, you may escape sandburs on your pant legs and pets for the whole year. For more information on crabgrass and sandburs, visit www.plantanswers.com.

Bermuda grass can make a lawn that looks like a golf course fairway, but it is also

one of our most prevalent hot weather weeds. The recent rains will turn it loose in our gardens and shrub borders that are in full sun.

            No weed is easy to control by hand-weeding, but Bermuda grass is especially difficult because every piece of root that remains in the ground or even on the ground will form a new plant. 

 

The "good news" is that there are a number of grass specific contact herbicides that are very effective in killing Bermuda grass in flower gardens, shrub borders, and groundcovers. Among the products are Fusilade II, Vantage, Over the Top, Grass be Gone, and Poast. Apply it to lushly growing Bermuda grass in the morning of a sunny day and within one week the grass will change color and stop growing. Within a few weeks the Bermuda grass will die.  The post-emergent contact herbicides kill the tops and the roots.

The herbicides are not effective on hardened off or dormant Bermuda grass; it must be green and growing.  Follow the label instructions closely for the best performance.

The herbicide labels do not allow the grass specific herbicides to be used on vegetables or bearing fruit trees, but they can be used around raised beds to prevent spread of existing Bermuda grass into the garden areas. 

Bermuda grass in a St. Augustine, zoysia, and buffalo grass lawns is a special problem.  The post-emergent herbicides for grasses also kill the other lawn grasses. In a St. Augustine lawn the tactic used to subdue Bermuda grass is to mow high, aerate, and provide enough water for the St. Augustine to prosper. Bermuda has more drought-tolerance and compact soil-tolerance than St. Augustine.

It is even harder to remove Bermuda grass from zoysia and buffalo grass. The only options seem to be hand-weeding or spot-killing with Round-up. The killed areas can be replanted with zoysia or buffalo sod.  They will fill in from the sides, but slowly.