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Can a holly tree ruin the soil?

The holly tree has been gone for about 10 years but was there for about 30 years. It stopped thriving when a nearby maple tree grew higher and cut out most of the sunlight for a large part of the day.

A robust 3' x 3' rhododendron planted in summer and showing new growth and blooms the following spring, died by the end of the summer. A replacement rhododendron (same species and size) is now showing the same trend. It too showed new growth this spring and had nice blooms, but it is also now beginning to wither and die.

The spot gets sunlight for about four hours each day. In the early part of the day the house blocks the sunlight and in the bottom hours of the day a maple tree puts the spot in shade. The nursery told me they would do fine under those conditions.

EDIT: We're in southeastern Pennsylvania not far from the Delaware state line.

The rhododendrons were watered during the summer every three days. The bush is about 10 feet from the maple tree which has a trunk about 18" diameter. The maple's roots spread out quite far and a large one passes within a foot of the root-ball of the rhododendron.

  • Bit more info please - what part of the world are you in, how close is the maple tree to the area, and did you water the rhododendron and its replacement during summer? – Bamboo Sep 20 '19 at 19:02
  • @Bamboo: I've added to the question giving some more info. – Tim Sep 20 '19 at 20:33
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No, holly does not make the soil inhospitable to other plants. The maple tree you mention, at 10 feet away, will be taking a lot of moisture from the soil, which means that anything new planted there will need extra water to try to survive. It's always difficult to establish a new plant within the root zone of a large tree, so if you want to try again, you'd need to give the plant a few gallons every other day - some of that will be taken up by the tree roots, unfortunately. If you can keep it healthy and growing for two years, its got more chance of surviving on its own. if you didn't do it previously, and want to plant another rhododendron, dig over an area at least three times or more bigger than the size of any hole you need to make and incorporate lots of well rotted or composted organic materials, such as composted manures or good garden compost. Let that settle for a week, then plant - the humus rich composted material will help to retain moisture.

One other remote possibility - a query as to quite what killed the holly tree - if it had been there that long, I wouldn't have expected it to die just because it was being shaded by the maple tree as it grew. Did you see any evidence of rot, or any mushrooms, particularly honey fungus which looks like honey coloured toadstools, usually around September, near the base? Honey fungus would mean that almost any woody plant you planted where the holly was would get sick and die because the mycorrhizae from it would still be in the soil, though I think, since the holly's been gone 10 years, this is a much less likely explanation for your rhododendrons dying than simple water shortage.

  • Thanks for the information. I don't recall any toadstools near the holly, though with the maple trees, lots of leaves got trapped between a fence and the house, covering the ground near the holly, and the leaves sit there, usually wet or at least damp, for quite a long time. The leaves have fallen and are on the ground for a good six weeks before the first municipal leaf pick-up. – Tim Sep 20 '19 at 22:42
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Trees have mycelium as helpers to gather enough water for them, and they share their sugars with the mycelium because all plants can only use about half of the energy they produce, so it's a symbiontic relationship. They also use this mycelial network to feed saplings, likely even from other species, but they also use this network to prevent some or a lot of other organisms from getting hold. Chestnut trees do this very effectively. I don't know about maples, but I could imagine that the advice of digging the hole 3 times as large (at minimum), filling it up with compost and fresh garden soil (so it doesn't contain the same mycelium since mycelium can start growing anew from just 1 cell if conditions are right), and then placing the new shrub in there. Indeed the first 2 years will be very important in it's battle for some land of it's own, if the Rhododendron's positive soil life can grow during this period, it will continue to support the plant and will probably also gain enough energy from the Rhododendron to defend itself against the different soil life of the Maple. Watering in time would be instrumental here, and would best be done not just for the Rhododendron, but also for the surrounding soil when not enough rainfall is present to support a moist/damp soil climate.

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Your soil could be the culprit, but not because of the holly tree.

Rhodies require both acidic and moist, well-drained soils (I've found that once established, they can take a bit of a drought though). What kind of soil do you have? Have you tested its pH?

Rhododendrons require acid soil (pH 4.5-5.5 is ideal). If your soil's pH is well above that, it's possible that the pH of the soil is actually killing your plants. This makes sense given that the rhodies do well for a period of time - presumably while the roots are still within the original root ball. When the roots move out into the soil outside the root ball, that soil's pH would be too high and the shrubs would decline.

Rhodies also do poorly in sandy or heavy clay soils. If you have clay and added compost, etc. to the planting hole without putting any of the clay back into it, you've essentially created an in-ground flower pot which will eventually either drown anything put into it or cause the shrub's root to circle the "pot" without going out into the wider world, harming growth. Note: I actually killed a rhodie this way.

  • The soil does have a lot of clay. I haven't tested for pH. Thanks for the info and for the very plausible theory. – Tim Sep 23 '19 at 13:45
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I have several American hollies up to 40 ft. tall and see no difference in growth under them compared to other trees. It is dark under magnolias so not much can grow there. I have a shallow pond from which I pull leaves out by hand and the holly leave thorns are as sharp as any cactus , so I suggest a rake if you are going to remove leaves. Black and Sugar Maples will make very dense shade ; In a forest preserve in IL, almost nothing grew under black maples except black maple seedlings and some early spring plants.

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