"Is it difficult to tell the poisonous ones from the safe? " No since they are all poisonous. The roots are very bad. But the leaves and stems are fairly poisonous too, they won't kill you, but you could be hospitalized. The berries also have poison, but in low doses. Easily removed if cooked. This goes for all elderberries. The red ones are said to be worse, but they are not. From Wiki: The ripe, cooked berries (pulp and skin) of most species of Sambucus are edible.[13][14] However, most uncooked berries and other parts of plants from this genus are poisonous. Sambucus nigra is the only variety considered to be non-toxic, but it is still recommended that its berries be cooked slightly for culinary purposes.[13] The leaves, twigs, branches, seeds and roots of Sambucus plants can contain a cyanide-inducing glycoside (a glycoside which gives rise to cyanide as the metabolism processes it). Ingesting a sufficient quantity of cyanide-inducing glycosides can cause a toxic build up of cyanide in the body. In 1984, a group of twenty-five people were sickened, apparently by elderberry juice pressed from fresh, uncooked Sambucus mexicana berries, leaves and stems. However, all twenty-five recovered quickly, including one individual who was hospitalized after drinking five glasses.[15] Such reported incidents are rare This post was edited by Drew51 on Sun, Mar 30, 14 at 23:13

yes i have seen this often with elderberry. even with well establishes patches with multiple plants, some of the berries will be great and some just dont quite make it. they take quite a while to get fully established before they produce a lot of flowers and berries. often they grow in huge patches with huge plants, you may want to add a couple of others of a similar variety to get better berries. and be patient, at least in my experience these take a couple of years to really get into their groove.

LOTS of misinformation going on here!! Elderberries - Sambucus species - are self fertile. You do NOT need two different varieties to get fruit but you will have much larger and more reliable cropping if you do. This is the same for any sort of self fertile fruit. These are caning shrubs - not trees - and do best in full sun and with reasonably moist soil. Raw fruits is perfectly edible but so astringent as to be virtually unpalatable. That's why most sources recommend cooking or otherwise 'preparing' the fruit, rather than eating fresh. Red elderberries (S. racemosa) are seldom used for berry production as they tend to be less palatable than the darker berried forms and reportedly are toxic if eaten out of hand (although they taste so bad fresh, I can't imagine anyone doing so!!). btw, as long as bloom times coincide, black elder can cross pollinate blue or American elder. Elder flowers are wind pollinated. And blue (S. caerulea) and American (S. canadensis) tend to be the tastiest and heaviest croppers of the various species.

Not sure about Elderberries, but with some plants - Blueberries being one - it's not that you need two different varieties it is that you need two that are not from the same gene pool. When buying blueberries, the easiest way to do that is to get two different cultivars (thereby, the two different varieties) because you know they are genetically different. I have a wild Elderberry and it does set some fruit, but not a lot. The following link does say for Elderberry: If growing selected clones for the fruit, you need to plant at least two different cultivars for cross pollination. Or you can have one named cultivar and one species plant. You might find this article helpful, it speaks about cultivars to some extent. It's not that you have to get cultivars, as I said that is just an easy way to know that you have plants with genetic differences. The real key is that you need your plants to bloom AT THE SAME TIME or at least have overlapping bloom times. If you don't, there is no way they can help each other. Here is a link that might be useful: Floridata