Elderberry stems and roots contain cyanide so I would say true. I grow all of mine in containers - probably could google it and find out more information

I am familiar with both red and blue elderberries native to the Pacific Northwest; they tend to grow on the edge of clearings and are often closely surrounded by other plants.

Elderberry is not allelopathic like black walnut.

My elderberry plants are surrounded by a variety of garden plants as well as the lawn. I do not see a toxicity effect in the vicinity of the elderberries.

Cheers!

I'm coming in late on this one, but I've noticed that nothing else will grow where the roots of my elder tree are. And I've tried LOTS of different things, even mints.

Elder is very common in hedgerows over here and it is invariably surrounded by a mixture of other shrubs and plants. I have a hedge which includes, apart from elder, hawthorn, sloes, dogwood, brambles and field maple. At its base grow daffodils, many grasses, stinging nettles, red campion, aquilegia, forget-me-nots and a lot of other wild flowers. So even if some things possibly don't like to be near it there are plenty of plants which couldn't care less. They thrive in heavy soils with plenty of moisture so maybe NSW is not their optimum habitat and they therefore take a lot of the goodness from the ground around them. (All this refers to Sambucus nigra.)

quote from movie "Miss Julie" (Froken Julie, 1951) :

162
00:36:48,199 --> 00:36:51,909
But I wanted to die
beautifully and painlessly.

163
00:36:52,004 --> 00:36:58,023
I knew sleeping under an elderberry
bush was fatal,

I moved two blueberry bushes from a location where they were doing okay, but were crowded by blackberries, to a new location where I prepared excellent acidic soil for them, and the new location is near, but not really crowded, by a large elderberry bush. I water them with rain water which I collect in barrels from roof runoff, no tap water. They are doing very poorly in the new location, getting smaller each year now for the last two years. The problem has to be either something in the shingles of the house roof getting into the rainwater that I collect from the roof or the elderberry bush. I strongly suspect that the culprit is the elderberry bush. I'm with Daisyduckworth on this one, but I'm ready to listen to any one who is that they know for sure. Oh yes, I forgot to mention that a volunteer blackberry seedling came along to the location near the elderberry bush and it s thriving. One more thing--berries and leaves from the elderberry often fall into the water barrel and decompose when the cover is off the rain barrel during rain catching times.