Accidental cross pollination

[Laura & Dave]

Hi all
I've been meaning to ask this for some time, as I'd like to keep
the various species I grow true.  How much of a problem is accidental
cross pollination between species in a garden setting?  I've checked the
archives and read Guy and Lilianes' book, but most of the information is
tangential.  There is quite a bit about deliberate crossing, which can
be useful in determining what crosses are possible.  And the bloom time
information is like wise valuable.  If crossing is a problem, what
measures can be used to prevent it?
The recent (great!) seed exchange prompted me to post these
questions, both to help assess seed I received and mostly so that when
the plants cooperate I can be confident that seed I pass on will be true
to their species.

Thanks
Dave Brastow
Tumwater, WA