Crushed lava rock (ugh!)

hostaLes(5)July 10, 2011

Reading back over the postings re: landscape fabric I felt I should pass on an experience I have had with red lava rock.

I didn't buy it for mulch but for curiosity - 1 bag only. If I ever get to post pics of my gardens you will note mostly bare ground. I find time in the shade using a shuffle hoe peaceful. After buying the lava rock I found myself in a position where I was going to be potting up lots of divisions of hosta for a new garden the following year. I decided to add a layer of the lava rock at the bottom of 4 larger pots for drainage.

The results: disasterous! Three of the four did not survive the following spring until planting time. Sickly little eyes opened and then nothing. When I tried to repot them and saw the rotted roots I threw them out. The single survivor was Sagae which was half the size of the potted division. Of 30 other divisions potted using my regular method I lost none.

That seems more than a coincidence and I consider the lava rock the cause of failure of the hostas to survive. I find it hard to believe the processor of the lava rock would go through the expense of adding a herbicide to it to control weeds. Do they do this? I was so irritated my customary curiosity was bypassed and I never checked pH of the soil. More probably the rock raised the soil pH way above what the tender hosta liked.

When potting up a lot of divisions like that I use drip coffee filters in the bottom. They are biodegradable and keep the soil in, and being designed to let water pass through when brewing a cup I don't find my pots waterlogged.

One of my SOLs likes everything neat and tidy. My daughters the gardener but he edges all the gardens and used the rubberized mulch on top that looks like red lava rock. I give it to him, it does look neat but I like hosta in a natural setting. I like all my gardens to look natural, which is hard with hosta when you need to leave so much space to grow out.


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ken_adrian Adrian MI cold Z5

what potting media did you use above the lava rock???

i am guessing soil was the problem .. if you didnt use a media ...

and i am further guessing.. if you used a media ... engineered for pots.. you wouldnt have been worrying about drainage.. enough to throw rocks in the bottom ...

drainage is drainage.. i dont think the rocks were the problem at all ... but you dont really give us any other info besides the rocks in the bottom ...


    Bookmark   July 10, 2011 at 9:33AM
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Ken - my potting media is a family secret (LOL) that even my German Shorthaired Pointer is forbidden to divulge, though she has tried to sell it. Seriously, i'd used the same mix in all of the pots (roughly 1:1:1 mix of my dirt: potting soil w/compost: sphagnum moss), so if all of the 30 prospered that I didn't use the rocks in (see my orig. post) I don't see why the mix would be suspect. I had 100% success in 30 pots without the rock, and 25% success in 4 pots with the rock. Could my pots with the rock been oversized, causing the propblem. And my original suspicions regarding the rock are still unanswered. The rock was used mostly to provide some bulk in the larger pots. I could have used styrafoam peanuts instead if I had them.

    Bookmark   July 10, 2011 at 5:19PM
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Adding rocks to the bottom of a pot moves the water table higher in the pot rather than helping it drain - so rot may have been from roots sitting in water.

Link below goes into why. From link - Plants that expire because of drainage problems either die of thirst because the roots have rotted and can no longer take up water, or they starve to death because they cannot obtain sufficient air at the root zone for the respiratory or photosynthetic processes.


Here is a link that might be useful: Container soils and water in containers (long post)

    Bookmark   July 10, 2011 at 6:11PM
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ken_adrian Adrian MI cold Z5

i dont know what pauls link says.. but i reduced my losses to nil.. when i quit putting mother earth in a pot ...

what works works ... and if you insist.. so be it..

i am just suggestion that it is SOME HOW .. at the root of your problem ...

e.g. most media is sterilized.. do you sterilize your soil before using it in your pots.. to insure you are not introducing unfavorable bacteria ... molds.. spores.. etc????


    Bookmark   July 10, 2011 at 7:52PM
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Shredded pine bark works for me! This year I am mixing in some gravel with the bark, as well as putting some of the gravel in the bottom of the pots (with a drainage hole) to hopefully deter voles!

Someone who knows a whole lot more about potting up hosta told me the pine bark doesn't even need to be shredded and I believe he also mixes in a bit of very good peat moss, too.

But, I've had good success with nothing more than the pine bark--shredded in my case!

    Bookmark   July 10, 2011 at 9:49PM
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I have read the posts on the container gardening forum and have read the same information in a gardening magazine, probably fine gardening. Both had the same conclusion, rocks on the bottom of pots interfere with the drainage. I had some problems with drainage in my large containers. After I read an article about how water does not move from the soil to rock, I stopped putting rocks at the bottom, a practice I started to save on potting soil. I have not had any problems since. The rocks I had been using where limestone landscaping rocks. I think the rocks are the problem.


    Bookmark   July 10, 2011 at 9:58PM
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Thanks - I think the rocks were the problem also. I never used them before, or since, and haven't had any significant losses. I do sterilize my pots cleaning them carefully with a bleach/water solution before using them. I just wasn't sure whether it was the lava rock or not. I guess it was rocks in general.

I probably wouldn't have used them at all except I ran out of smaller pots and was trying to save on potting mix. Last year I bought a huge bag of a bark mix from my home-town nursery. It is what he uses in his pot-ups. It is hard not to notice that when you buy a potted plant there are no rocks, chips etc. in the bottom. I probably wouldn't even use the coffee filters except with 30 or more plants they are everywhere I don't want potting mix running out. Generally I avoid using dirt, but I tried to conserve $ spent on a prepaired mix. My thinking may have been flawed since I was just housing dormant plants during the cold of winter when the mix would have been biologically dormant also in freezing temperatures. I felt I was just trying to keep them from dessicating.

Paul - that was a great link. I forgot to mention my soil was infused with a lot of pearlite. This wicking makes sense. The potting mix with its small particles has greater surface area than the rock so gravity is overcome by the wicking, thereby retainging water. Since I was simply overwintering the plants they were dormant and not using the wicked water up, thereby they became water logged. duh on me.

Ken-I don't "insist" on any such thing. I only used the rock in 4 pots out of 34, and have overwintered a hundred or more in the past without rocks and with little loss. It seems the problem was using rocks in general rather than specifically lava rocks. Needless to say I haven't used rocks since.

    Bookmark   July 11, 2011 at 12:45AM
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Well, ya had me worried about my pots and whiskey barrels with a layer (or two) of granite rocks in their bottoms, so I went out and poured water into one of my potted up hosta and it poured straight through out the bottom with no problem,! Whew!

I wonder if the lava rocks, (by design rough and pitted) would not be more inclined to not slough off the water, whereas granite, being smooth surfaced has more ability to allow the water to slide off? Combine the lava rock with your soil recipe, which might be more inclined to hold water, and perhaps it was just a deadly combo. Around here granite is used around alot of man-made ponds and lakes to keep the levels from overflowing their banks--at least that's what I'm seeing!

    Bookmark   July 11, 2011 at 9:05AM
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Pieter zone 7/8 B.C.

Yes, Janice, water will run straight through the rocks at the bottom. The problem however is with the retained water in the soil, the so-called perched water. It stays in the planting medium longer because the water does not easily migrate from the small particles to the much larger rocks. Rocks or any other material at the bottom of containers reduces the amount of space available for critical gas exchange by the roots and also reduces the amount of space available for root development. To prove this to yourself, take 2 quart sized semi-transparent foot containers, drill holes in the bottom, fill one completely with potting mix, the other with a layer of rocks topped up with the same potting mix. Pour a quart of water into each container and let them drain. Leave them sit for 3 days and then feel how deep you have to go with your finger to feel wet medium, not moist, but wet. You will find the container with the rocks has a noticeably higher water table, which leaves roots sitting in water, leading to root rot and eventually crown rot and death.

Next time you refresh the planting mix in those containers, remove all the rocks, they have no business being in there.


    Bookmark   July 11, 2011 at 5:50PM
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