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I'm thinking of putting decorative white pebbles on top of my potted outdoor plants. I googled this, and some articles that I found raise several concerns:

  1. Pebbles in soil increase soil pH, which is not always desirable.
  2. They make the soil warmer in summer, which might harm the roots.
  3. Soil with pebbles on top retains moisture better (because it would be harder for the water to evaporate), which is also not always a good thing.

How valid are those concerns? Can decorative pebbles actually do harm?

I found this answer https://gardening.stackexchange.com/a/19392/25919 that addresses the 3rd point.

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  • Point # 1 assumes the pebbles are limestone ; I often see granite decorative pebbles ( little or no pH affect ). – blacksmith37 Mar 31 at 0:36
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    White pebbles will have a much higher albedo (reflectance) than potting soil, so I'd expect them to cool the pots. – Wayfaring Stranger Apr 1 at 14:29
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Point one and two depend very much on the type of pebbles you use.

  1. The pH change should be negligible for most kinds of gravel isn't made from carbonate rock (like limestone).

  2. The temperature problem has two sides: first depending on their color the pebbles can draw heat which could be problematic for the roots. That would be a problem for dark gravel though not so much for white pebbles. You would get the same effect with a black pot. On the other hand, they can have a higher heat capacity than the soil and help stabilize the temperature from spikes. I wouldn't consider heat a problem with white pebbles.

  3. This is true for every kind of mulch which is usually one of the reasons they are recommended. For a pot, you run into the problem that it usually dries out quick, but it is also prone to become waterlogged. So watering pots is something you need to watch out while doing anyway. This shouldn't be a problem with some adjustments.

So don't use limestone, don't use anything black and keep an eye on watering, and you shouldn't need to worry about the points. Maybe look at quartz gravel for example. On the other hand of course there are plants that like alkaline soils, their roots hot or the soil wet. So adjust as needed.

Edit:

Regarding temperature: You probably get the opposite problem in spring: the soil will probably heat up slower than it would without the gravel mulch. The gravel will provide isolation and - depending on the material you use - heat up slower than the soil would.

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Assuming the pebbles you use are not limestone, there are two possible problems; first, if the plants are in full sun, stones, whatever type they are, get hot in sun and retain heat for longer, even after the sun has moved away. Second, one of the ways to monitor watering requirements is by the appearance of the soil on the top of the pot and by touching it - this is not so easily done with stones in the way, unless they are medium sized pebbles which can be pushed aside momentarily.

Organic mulches such as bark chips or composts, on the other hand, do not get overly hot in direct sun and nor do they hold heat when the sun has gone.

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