Question
I have planted several perennials near in front of some large lilacs in my yard.  These are mostly shade perennials (various hosta cultivars, wild geraniums, blue cohosh,jack in the pulpits).  Many of these plants have failed to thrive in this location although plants of the same species grow well in other areas of my yard.  I'm beginning to wonder if the lilacs are simply out-competing the perennials and are sucking up all of the nutrients that the other plants need.    Do lilacs have "greedy" root systems?

Answer
Hi Vince,
Thanx for your quetion.  I was doing some research on the Internet on the allelopathy of lilac.  Allelopathy, simplistically, means sensitivity of other plants to one particular plant.  A classic example is the allelopathy of black walnuts.  If you know black walnut trees you know that little or nothing will grow underneath the tree or near its roots.  That's because the tree exudes a chemical called juglone which is poisonous to most other plants but there are some plants that are not affected by it.  It appears that lilacs may do this too with certain plants.  I couldn't find a list but if you have plants that are not performing well or at all, I say move them.  I don't think it has anything to do with "greedy" root systems.  The roots may be exuding some kind of chemicals your other plants find disagreeable.  I hiope this helps.
Tom