Detrimental? No. But larger shrubs/trees that are well-established will have wide spreading root systems that can sometimes out compete smaller, newer plants for soil moisture and nutrients. Sometimes just a little extra TLC (a bit of fertilizer, more water) directed at the smaller plants will offset this easily.

I've rotated many hydrangea macrophylla in the vicinity of 3 yews. It seems that the yews sap soil nutrients away from the hydrangeas causing them to not bloom much. My two cents...

That's it, springwood. That's what I think is happening. I can't believe I have to more all those hydrangea and hosta now.

Thank you so much.

Pat Z5/6 SE Mich

But, gardengal, I have about 13 oaks, so I'm not concerned about roots. Have you any knowledge about the toxicity of yews in particular? I have heard that the yew needles are poisonous to animals.

But it's the roots that are causing the problem. Yes, the yews are toxic if YOU eat them (livestock as well) but they have no effect on the hydrangeas as far as toxicity goes. Yews are juxtaposed with other plants in the landscape all the time. Pruned yew hedges surround vast perennial and shrub borders in England. There is NO issue with the yews poisoning other plants.

If you are talking about aleopathy-- that is when a plant sends out toxins in the soil or leaves or nuts (nut trees in particular are aleopathic) to prevent other plants from doing well. What the OP has described is competition for nutrients of an established tree/shrub vs. newer plantings. Gardengal48 is correct in providing more water and fertilizer to help those newer planting establish better.

One more vote for giving them more fertilizer and water. You have the old, established yews with lots of roots taking in most of the available water and nutrients. If you add more water and nutrients all your plants will have enough. I am not often an advocate for supplemental fertilizer as it is often unneeded, but this is a classic case where it will make a world of difference. Don't water by just sprinkling for a few minutes with the hose. Water long and deep with a soaker hose or a low-volume sprinkler. Adding a light layer of mulch will help keep the moisture levels more even between waterings and add a few nutrients over time as well.

NHBAbs, thank you so very much for that advise. I do love to deep water my hostas and hydrangeas and I will definitely start fertilizing them more this year. In the past it was thought that fertilizing plants under a tree/trees meant that the tree roots would rush to the plant to get the fertilizer, but I think with proper watering/fertilizing both should get what they need. I'm 74 this year and so wish I had thought about this years ago. Again, thank you.

Golly....I could have sworn I provided the exact same advice - extra water and fertilizer - in my initial response to your posting :-) Oh well.........

gardengal, you did indeed provide that exact same advise and I apologize for not acknowledging you also. I've had my head on wrong for a few days now. You are always one of the people I look forward to learning from on GardenWeb.

Pat