Re: How do plant hormones are used to control unwanted plants(weeds)?

Plant hormones, or really chemicals that work like plant hormones, can be 
used to control unwanted plants.  Specifically the plant hormone known as 
auxin (abbreviated as IAA) and chemicals that act like auxin are used in 
various types of weed killers.  Several different types of chemicals that 
act like auxin are in weed killers you can purchase at a garden center or 
place where garden supplies are sold.  Some of the chemicals that have been 
used in the past or present include 2,4-D; 2, 4, 5-T (contains dioxin), 
picloram, and dicamba.  There is a lot of work to try and understand how 
auxins and auxin-like chemicals work to kill plants.  It is very clear that 
these chemicals work much better on dicot plants (plants that have two 
cotyledons and are sometimes known as broadleaf plants) than on monocot 
plants (plants with only one cotyledon).  The monocots include grasses and 
these herbicides are often used on lawns and in fields where corn, rice, and 
wheat are grown.  Corn, rice, and wheat are types of grass.

So how do these chemicals work?  It isn't totally clear, actually.  It seems 
as though these chemicals are taken up by dicot leaves and are transported 
in the plant much more easily than in monocots.  We know that plants that 
are exposed to auxins and herbicides containing auxin like chemicals have 
changed growth patterns.  Certain parts of the leaf and stem begin to grow 
more quickly and others don't grow as quickly.  The result is twisting of 
the plant parts.  These types of growth patterns often result from a second 
plant hormone ethylene.  Auxin increase the amount of ethylene the plant 
produces.  But there is probably more to the story than just differences in 
uptake of the chemicals and increased ethylene production.  The hypothesis 
is that the chemicals stop the production of chemicals important for proper 
plant growth.

Source
Salisbury and Ross.  1992.  Plant Physiology, 4th edition.  Wadsworth