"Doug Kanter" wrote in
:

I have a pine tree at my new house. There's nothing under it but dried
needles, and some hideous red mulch applied by the previous owner.
Now, I've spent enough time hiking to notice that in the woods,
there's not much that grows under pine trees. But, that's before WE
get involved. :-) The circle under mine begs for something, whether it
be just green like pachysandra or vinca, or maybe a bunch of spring
bulbs. Is it hopeless? Do the needles wreak havoc with pH to the point
where the soil's unfriendly to most plants?

Let's assume for the sake of discussion that if lack of water is the
issue, I'm willing to correct that. I've got this new high-tech thing
called a sprinkler.

aside from physically blocking light, it's been rumored that pine needles
contain/produce some phenolic compounds that inhibit the growth of other
plants. of course there are some plants that don't mind living in a
tough neighborhood and so you've got your strawberries and such thumbing
(hehe, he said 'thumb') their noses at the pines and wussy plants.

this is a from 1994 not sure it really says anything other than it needs
further investigation

http://www.colostate.edu/Depts/Entom.../en570/papers_
1994/carroll.html