We put down red lava rock in a 4x30 bed last year instead of the annual red pine mulch that bleached out and burns up in the Florida sun. We completely removed the mulch, dug down about a foot and removed any weeds. Put down one of the most expensive landscaping cloths, not black plastic, as there were already plants in the bed. Black Edging and 3-4 inches of lava rock put down. Within 1 week there was St Augustine grass coming up thru the fabric and rock in the center of the 4 ft bed. The stuff is a nightmare to try and maintain. You’ll destroy the fingers of any garden gloves weekly. It DOES RETAIN HEAT and my plumerias lose leaves daily from being baked. It’s also breaking down, with no car or foot traffic in the bed and there’s almost an inch of red rock sand now trapping weed seeds also. We’re in the brutal heat/humidity part of the year right now. In a couple of months, we’re removing it all. Not sure what the replacement will be.

njoasis, you've posted this twice. See my answer to your previous post on the same subject. Patty S.

Don't use fabric. And don't use rocks. I would use only mulch- it degrades and nourishes the earth, it keeps moisture in the soil, and helps keep roots cool. Plus when you refresh it every year or so it makes everything look all new again. Rocks, once put down, are forever and someday somehow you will want them gone and be unable to gather them all up. You'll be forced to hire neighborhood kids for a penny a rock. They just become unsightly and bake everything underneath. As far as the fabric there are varying viewpoints but I do not find that it suppresses weeds all that well (birds still drop seeds that germinate on top) and there is evidence that it disrupts gas exchange and beneficial soil organisms. Roots need air.

Red lava rock, or red-lava-sand, is a great source of iron and potassium. pH of red-lava-rock is 8.2. Red-lava rock is OK like 3 or 4 pieces, too much will UP the soil pH, plus prevent water from soaking through. I went to my neighbor when it was above 90 F one summer. She mulched her roses with THICK layer of black lava rock. I touched them, and they are the same temp. as our rock-hard-clay. She overkilled on the lava-rock: too much potassium drove down nitrogen, so her roses didn't have much leaves, lots of blooms. I would use no more than 3 pieces per rose .. too much and one gets iron-burnt (brown-spot). http://forums.gardenweb.com/discussions/1539970/lava-rock-as-mulch ocdgardener(8) From what I understand about Lava Rocks is: Lava rock breaks down into Lava Sand. From all the sand in the bags I know it happens! Lava Sand is recommended as a soil amendment to HOLD MOISTURE. Now, the only thing I'm not sure about is temperature. I did notice though oddly enough last night when I got home from work the plants in the same bed mulched with cedar were wilting, the plants surrounded by lava were not. Here's the positives I've found thus far. On a slope where my bed is, compost, and every thing else added at the base of the plants washes down to the bottom and is replaced with bare soil. I have to add mulch over and over because it all slides away. The Lava Rocks are not only holding the compost in place but are keeping me from having bare soil. I know it does because we had a heavy rain this morning and I looked and everything was in place! --Not the usual bare soil I see after a rain! I think I should explain how I'm using the rock. I'm taking one rock at a time and pressing it into the soil. When you think of lava rocks as a mulch I think most people tend to imagine it piled up. ocdgardener(8) oh and btw you can't really see that much of the rock anyway - because the plants cover them. Example: This is what the yard looks like in the spring. ocdgardener(8) Update on the lava rock! I bought a soil thermoter and tested the temps. Although not lower, the temps were pretty close to the same by a degree. Also, I have calendulas blooming in the bed in the HEAT of summer. I've never had that before! mrwsm_yahoo_com Lava Rock has low thermal mass. It does not retain heat. Lava Rock keeps the soil soft like mulch, so if there is a weed, no problem, it pulls really easy. I prefer red lava rock, although I have both. Never put plastic underneath it! It keeps most weeds out as is. rickjones I don't know what happened to my original post, but here is a short repeat. LAVA ROCKS ARE WONDERFUL PLANTERS, ESPECIALLY IN THE EXTREME HEAT OF THE MOHAVE DESERT. Yup. I build and use cedar planters and window boxes all over my home, they are great, but do not compare with the flowers grown in the lava rocks that I hollow out and plant a variety of species in. Plants in these rocks see temps to 120 degress in our summers and do just fine! They retain moisture, cool with the air temps, and fertilize plants naturally. I don't use lava rock for bedding, I can see the problems with that. But as a planter? The best planter I have ever had. AND, no maintenance, and years of use looking great. jocoreed_yahoo_com Anyone who questions the ability of lava rock as a growing medium needs to come visit our islands out here in the Pacific. Our Hawaiian islands are completely volcanic is soil and sediment, covered in pure lava rock, and we have the most beautiful plants, trees, produce, and the like. Lava rock and volcanic soil is some of the most nutrient friendly planting medium on earth and is porous enough that it allows for good oxygenation and water flow. Come visit our islands and then come back and post how it's horrible to grow anything in volcanic rock or lava sand. Just my two cents : ) Aloooooha! http://forums.gardenweb.com/discussions/1539970/lava-rock-as-mulch