Here's one bank that won't default

Just a quick note about soil seed banks. If you are not familiar with the term, it is where seeds are stored in the soil until conditions are appropriate for germination or where they decompose.  There are thousands of seeds in a seed bank and not all will germinate at any given time or any given set of conditions. That's one of the reasons annual weeds are a problem year after year, even if you kill the current year's weeds before they produce seeds.

If you need proof, I'm attaching a photo of a plot that was tall fescue turf for the past 5 years and there were few weeds in the plots.  I then shut off off the water for a few months and the fescue died. I recently started irrigating a few plots and up came a bumper crop of common purslane and spotted spurge.  

Such a heavy infestation would not have come from nearby sites so all I can think of is that the seed bank stored seeds from many years ago.  The turf was competitive enough to suppress most weeds so any seeds were not successful in establishing.  

So here are two ecological principles being demonstrated: the role of the seed bank in maintaining weed populations and the role of interspecific competition in suppressing weed growth and establishment.