Lava rock doesn't provide any nourishment for plants.

Can You Grow Edible Herbs in Lava Rock?

by Cindy Quarters

Lava rock is a common mulch and landscape material but isn’t much good as a place to grow plants by itself. It does have some properties that make it useful in hydroponic systems, and when used in combination with other hydroponic essentials, lava rock makes a safe and solid growth medium for edible herbs. Not only that, but a hydroponic system will let you grow herbs for your family all year, no matter where you live.

1. About Lava Rock

Lava rock is inert and doesn’t provide plants with any of the nutrients they need, but it can give them plenty of physical support. Use it in place of other media in various types of hydroponic systems. According to a study by Iowa State University, lava rock doesn’t alter the pH of its environment significantly. It does radiate back warmth absorbed from the sun or other heat sources and effectively retains moisture, lowering the risk of losing hydroponic plants if there’s a temporary system failure.

2. What Plants Need

Plant nutrient requirements change as they grow and enter different stages of development. As long as you supply the basics, your herbs will take what they need. All plants need certain elements in relatively large amounts. These macronutrients include nitrogen, calcium, potassium, sulfur, magnesium and phosphorous. They need only trace amounts of certain other elements, called micronutrients. These include chlorine, zinc, manganese, molybdenum, iron, boron, cobalt and copper. Hydroponic plant foods include all of the nutrients necessary for plants to grow and develop properly; nothing comes from the lava rocks.

3. Growing Herbs

Try planting different herbs in pots or trays filled with chunks of lava rock, then water the herbs according to their basic requirements. If you can grow the herb in a garden, you can grow it in your hydroponic system. Herbs thrive under the same general conditions they prefer when planted in soil: mint does well with plenty of water and cool conditions, while herbs such as basil like warmer weather and wet media. Let the lava rock dry out a bit between waterings for herbs like oregano and basil.

4. Using Lava Rock with Hydroponic Systems

Not all hydroponic systems or growing situations lend themselves to the use of a support medium such as lava rock. If you want to start your plants from seed, choose perlite or rock wool, since most lava rock is far too coarse for seeds. Small chunks of lava rock can replace other types of media such as pea gravel or perlite in the net pots commonly used in deep-water culture systems. It also works to fill the growing trays typically used in ebb and flow systems and makes a useful support medium for single-bucket drip or hand-fed systems.

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