Growstones hydroponics means growing plants in Growstones, a lightweight, porous aggregate made from recycled glass. It holds more air than perlite, wicks and holds water inside its pores and can be reused. Research found Growstones produced tomato and cucumber yields equal to rockwool, with a drier root zone and less Pythium. The main catch: new Growstones can push water pH above 10, so rinse and soak them before planting and check pH for the first few weeks.
Key takeaway: Use Growstones in ebb and flow, drip and wicking setups or as a perlite substitute in mixes. Rinse well first, water in short, frequent cycles and expect a slight pH rise early on.
| Property | Growstones | Clay pebbles (LECA) | Perlite |
|---|---|---|---|
| Made from | Recycled glass | Expanded clay | Expanded volcanic glass |
| Shape | Irregular, rough chunks | Round pebbles | Small white granules |
| Wicking | Good | Fair | Good |
| Floats? | Can float when dry | Can float when dry | Floats easily |
| Starting pH | Can be high until rinsed | Near neutral after rinsing | Near neutral |
| Reusable | Yes | Yes | Limited |
What Are Growstones?
A 2011 HortTechnology study by Evans and Jackson at the University of Arkansas describes how Growstones are made. Finely ground waste glass is mixed with calcium carbonate and heated in a kiln. The heat releases carbon dioxide as the glass particles fuse, trapping air spaces inside. After cooling, the product is ground to the desired particle size. The result is a light, foamed recycled glass with many internal pores.
The same paper notes that Growstones had already been used successfully as a hydroponic substrate and had been proposed as an aggregate to adjust peat-based mixes. Dutch researchers describe the material as a “glass foam granulate” in particles of about 0.2–2 inches (0.5–5 cm).
How Growstones Hydroponics Performs
Air and water
Evans and Jackson found Growstones had a higher air-filled pore space than both sphagnum peat and perlite. Adding at least 15% Growstones to peat raised air space, and mixes with 20% or more Growstones held more water than equivalent perlite or parboiled rice hull mixes. Growstones mixes were also heavier than perlite mixes. Plants grown in these recycled glass mixes were similar to those in perlite and rice hull mixes.
In straight Growstones slabs, a study by Blok, van Winkel and Chizhmak (Acta Horticulturae, 2011) found the coarse granulate held a maximum moisture content of about 50%, but rehydrated very fast, reaching more than 80% of container capacity in 5 minutes. That is why the authors recommend small, frequent irrigation cycles.
Yields
In those Dutch trials, cucumbers on Growstones produced as much as plants on rockwool, with the first harvest 1–2 days earlier. Tomato yields on the glass granulate and rockwool were also equal. The researchers found significantly lower susceptibility to Pythium on Growstones, which they tied to the drier root zone, and tomato plants recovered faster from blossom end rot. They describe the material as “more generative,” meaning it nudges plants toward flowering and fruiting.
pH and Chemistry: The Big Growstones Hydroponics Caution
Blok and colleagues report that the material initially reacts with water and can raise pH to over 10. Based on lab tests, they calculated a rinse recipe and concluded the material must be rinsed with an acid solution before planting. They also found small amounts of silicon released into the nutrient solution, which made cucumber fruit wax noticeably thicker.
Michael Evans at the University of Arkansas tracked mixes with 10–30% Growstones for 8 weeks. pH rose as the Growstones share went up and over time, from 5.7 to 6.9, still within acceptable limits. Sodium also increased with more Growstones but stayed in recommended ranges, and nutrient levels were acceptable. For pure Growstones hydroponics, where there’s no peat to buffer the change, the pH effect is stronger, so rinsing matters more.
How to Prepare Growstones for Hydroponics
- Rinse the dust. Put Growstones in a bucket or mesh bag and rinse until the water runs clear.
- Soak and check pH. Soak in clean water for several hours, then test the soak water. If it reads high, drain and repeat, or soak in water adjusted down with pH Down. See our media pre-rinse and buffering protocol.
- Pre-wet before planting. Dry Growstones can float in a flooded tray. Soaking lets water fill the pores.
- Plant rooted starts. Transplant seedlings in rockwool cubes, plugs or starter sponges into the media.
- Monitor pH for the first weeks. Check the reservoir daily at first and adjust to your crop’s target range.
Best Systems for Growstones Hydroponics
- Ebb and flow: Growstones drain fast and hold air, which suits flood-and-drain cycles. Shorter, more frequent floods fit the drier nature of the media. See ebb and flow hydroponics.
- Drip and slab systems: This is how the Dutch trials grew tomatoes and cucumbers, with small, frequent irrigations.
- Wick and self-watering pots: The pores wick moisture upward, so Growstones work in passive and semi-hydro containers.
- Mixes: Use 15–30% Growstones in peat or coir mixes as a perlite substitute, as in the Arkansas trials.
- Less ideal: Deep water culture, where you only need a small amount of media in a net cup.
Growstones Hydroponics vs Clay Pebbles and Perlite
Clay pebbles are round, so they roll and pour easily, but each pebble has fewer wicking pores. Growstones’ rough, irregular shape locks together in a pot and pulls water higher. Compared with perlite, Growstones are chunkier and less likely to wash out of flood trays, and they don’t break down as fast. For a full comparison of media, read growing media for hydroponics and clay pebbles (Hydroton).
Can You Reuse Media From Growstones Hydroponics?
Yes. Because it’s glass, the media doesn’t decompose. After a crop, shake out roots, soak and scrub the stones, then sanitize and rinse before replanting. The Evans and Jackson paper frames Growstones as a recycled product made from waste glass, which is a plus if you want to avoid mined media. As with any reused media, discard batches that held diseased plants or sanitize them carefully.
Growstones Hydroponics Troubleshooting
- pH keeps climbing: The media is still releasing alkalinity. Rinse again or soak in pH-adjusted water, and check the reservoir daily until readings settle.
- Top layer stays dry: That’s normal and helps keep algae and fungus gnats down. Make sure roots below are moist by checking a few inches down.
- Plants wilt between floods: Growstones drain fast, so in Growstones hydroponics you may need shorter gaps between irrigations, as the Dutch researchers advised.
- Cloudy water after setup: Leftover dust. Rinse more before the next fill.
FAQs
Is Growstones hydroponics a good choice for beginners?
Yes; it is forgiving, and studies found tomato and cucumber yields in Growstones equal to rockwool.
Do Growstones raise pH?
New Growstones can push pH above 10, so rinse and soak them before use.
Can you use Growstones in DWC?
You can, but a little media in a net cup is all DWC needs, so cheaper options work too.
Are Growstones better than clay pebbles?
They wick better and stay put; clay pebbles are easier to pour and rinse.
Do Growstones float?
Dry pieces can float; pre-soak them so the pores fill with water.
Can Growstones be reused?
Yes; clean and sanitize them between crops.
Sources
- Evans and Jackson (2011), HortTechnology — Physical Properties of and Plant Growth in Peat-based Root Substrates Containing Glass-based Aggregate, Perlite, and Parboiled Fresh Rice Hulls
- Blok, van Winkel and Chizhmak (2011), Acta Horticulturae — Glass Foam Granulate as Growing Medium for Tomato and Cucumber
- Evans (2009), ASHS Annual Conference — Chemical Properties Over Time of Root Substrates Containing Increasing Amounts of Growstones, Perlite and PBH
- Photo credits (Wikimedia Commons, edited with titles): Gesneriaceae, Growstone by Gesneriaceae in Thailand (Public domain); Hydroton by Lucis (CC BY-SA 3.0); Perlite1 by KENPEI (CC BY-SA 3.0); Rockwool cubes-inlay PNr°0091 by D-Kuru (CC BY-SA 3.0 at); Hydroton by Lucis (CC BY-SA 3.0)