The following information gives a basic understanding of Growstones:

1. What are Growstones made from? 
  Growstones are made from recycled glass beverage containers received from either the landfill or another source collecting and processing waste glass. The glass containers are crushed into small glass pieces and then ground into a fine powder (consistent with the size and the feel of flour). This “glass powder” is the main component of Growstones. A natural foaming agent is then added to the “glass powder” before it is heated and expanded in the Growstone kilns. To learn more, go to Manufacturing Process.

2. What is the composition and final appearance of Growstones?  Growstones aggregates or stones are made from approximately 98% recycled glass and about 1.25 % of calcium carbonate, a natural foaming agent. The final product are rigid and highly porous foamed glass material, which is crushed into different particle sizes of irregular shaped aggregates or stones.

3. Do Growstones allow for capillary water rise (wicking capacity)? 

Yes. Growstones wick up water up to 13 to 15 cm above the water level. Since capillary rise is driven by pore size and structure, the pore size inside each aggregate and between each aggregate, determines how high water can rise in Growstones. The smaller the aggregate (stone) size, the smaller the inter-aggregate pore space and the higher the capillary rise.

4. What are the physical characteristics of Growstones?

5. What is the importance of bulk density of hydroponic substrates?

For all substrates, one of the most important aspects to consider for root growth is the availability of oxygen inside the substrate after irrigation. The bulk density is important due to its distinct relationship with total porosity and oxygen availability in root zone: the lower bulk density, the higher the total porosity. This equates to more aeration for the roots.

Optimum air content for root formation and growth in greenhouses is at least 20 %, and can be as high as 45% (Gislerod, 1982). Growstone products maintain between 48 and 51% of air space at saturation (after all free water has drained away). This facilitates gas exchanges between the roots and the environment around them. Growstones reduce opportunity for anaerobic conditions that lead to root rot and lead to higher oxygen/lower carbon dioxide levels – a healthy environment for root growth and development.

6. How do Growstones moisture/air characteristics compare with other hydroponic substrates?

The graph below compares different hydroponic substrates regarding water holding capacity and air-filled porosity at saturation. At saturation (after all free water has drained away), most substrates enhance either water retention (left on graph) or aeration (right on graph), but not both. Growstones and perlite have a great balance between the two.

Growstones maintain a very unique balance between moisture (30%) and air (50%) by volume.


The advantage of Growstone vs. perlite is a much higher aeration, which is not accomplished at the expense of a decrease in water holding capacity.

The advantage of Growstones vs. Hydroton is the clear increase in water holding capacity due to Growstones typical open and interconnected pore cell structure, which can absorb and hold between about 30% of its volume in water by capillary action. What is really important and effective is that in Growstones, the higher air-filled porosity is not accomplished at the expenses of reducing water holding capacity, about 50% by volume. Another great advantage is that Growstones release considerably less fines than perlite or Hydroton.

7. Do Growstones facilitate grower crop management and plant steerability? 
Yes. In greenhouse horticulture, steerability is a measure of how easy it is to move the plant towards a more reproductive or a more vegetative growth pattern. This is mostly accomplished by controlling root zone moisture content. Because Growstones very high aeration is associated with a reasonable capacity to hold water, the root zone can be as dry or as moist as the grower wants it to be, and the desired root moisture content can be achieved quickly. Quick and easy control over root moisture makes it easy to ‘steer’ the crop and to reach a balance between the two growth patterns. Plants grown in Growstones respond fast to changes in irrigation scheduling and climate.

8. What is the impact of Growstones drier nature compared with other media?

Besides easy to steer the crop into a balance growth, Growstones drier nature also help suppress root diseases such as Pythium. Also allows for faster recovery from human mistakes that lead to Blossom end rot (BER) for example. These characteristics facilitate crop management compared to substrates with high water holding capacity which water content takes longer to change.

9. Can Growstones be reused? 
Yes. Growstones keep their structural integrity over time, so it can be reused multiple times. As with any other substrate, sterilization between crops is recommended. When available, steam sterilization is the ideal option. With steam, a cubic yard of moist GS-1 Hydro Stones reach 200 ⁰F in less than 20 minutes without the use of a blower. This temperature can be maintained for 15 – 20 minutes after steam is turned off. So one cubic yard (27 ft3, about 174 gallons) of Growstones can be completely sterilized and ready to be reused in 35 – 40 minutes.

10. When used in mixes do Growstones float and wash away with irrigation? 
No. Growstones do not float to the top of the soil and wash away with irrigation. This is due to its and irregular surface, which anchors the aggregates to the bulk of the mix.

11. Are Growstones available in different particle sizes?

Yes. Currently Growstones are available in three particle size ranges. For certain applications Growstones work best with specific particles sizes. The following are the different particle sizes recommended for different applications:

a. Small for fungus gnat control – between 0.157“ – 0.025” (4 to 0.63 mm).

b. Medium for soil enhancement – between 3/8”- 0.157” (9.5 to 4mm)

c. Large for hydroponic growing (alone or in blends) – between 3/4″- 3/8″ (19 to 9.5 mm).

12. How does one use Growstones? 

This question must be answered in two parts, because Growstones can be used in two very distinct and different ways.

The first and most simple way to use Growstones is as a soil enhancement for the cultivation of potted crops, container gardens, flowering beds and/or landscapes. Simply mix Growstones GS-1 Hydro Stones or GS-2 Mix This! into your mix at a 20 to 30 % ratio depending on plant, watering requirements and climate. See GS-2 Mix This!  Soil Aerator page for specific information on ‘How to Prepare Your Growstone Planting Mix’.

The second way to use Growstones is by itself as a hydroponic growing media. See GS-1 Hydro Stones Hydroponic Substrate page for instruction on ‘How to Prepare Growstones for Hydroponic Use’.

13. Has anyone else tested or used Growstones? 
 Growstone has invested time and money in testing and developing its products. To learn more about results from Growstones trials in greenhouse hydroponic production, biofiltration in aquaculture or green roofs, go to Research & Field Trials.

Growstones have also been used in large commercial greenhouse operations in Texas and Mexico (Imuris) with similar results regarding yields and crop development compared to the standard substrate used in each operation.

14. Where are Growstones made? 
Growstones are manufactured in Albuquerque, New Mexico. Our manufacturing plant is located at the City of Albuquerque landfill where we take the recycled glass right from the recycling trucks as they are unloaded, washed and crushed.

15. Are Growstones Organic? 
  The simple answer to this question is no, Growstones are not organic. The detailed answer to this question is that while Growstones are not organic due to the process of making glass bottles, Growstones can play a vital role in the green movement.

Growstones can fit into a sustainable agriculture/horticulture program and are definitely part of the foundation of the green movement: Reduce. Reuse. Recycle.

16.  Can Growstones be used with worms?
Earthworms need lots of moisture and organic matter, so if you use worms to grow, Growstone blends are the most suitable choice. Worms do well in well watered Growstones blends. Despite the fact stones might seem coarse to human touch, all evidence points to the fact worms follow organic matter and water and Growstones do not get in their way.

17. Can I use Growstones instead of Miracle-Gro? What about the nutrients from the soil? Do Growstones provide nutrients? 

Growstones GS-1 Hydro Stones and GS-2 Mix This! can be blended with any type of soil/soilless media to add extraordinary aeration, fast drainage and measurably reduce compaction of soils, promoting aerobic conditions for root health. It can and should be blended with high water content soils such as Miracle-Gro, coco coir or any other blend. Growstones leach out soluble Silicon over time, but other all nutrients will have to be added in the fertilization regime as Growstones do not provide nutrients.  New Growstone blends containing a complete nutrient charge will be available in 2015: Growstone Planting Mix and Growstone Seed Starter.  Grow out trials showed these mixes outperformed Miracle- Gro shortly after germination giving seedlings a significantly stronger head start.

18. How can one purchase Growstones? 
For a listing of Growstones distributors and to find out where to buy Growstones near you, go to Where to Buy.