Hydroponics Cultivation Tips

The water retention of artificial substrates is an often overlooked, yet important factor, to consider in grow room design. This is a fact in vertical farming and aquaponics operations, as well as cannabis and food production. Modern gardeners choose their cultivation mediums based on several factors. These factors mainly have to do with operational logistics, fertigation programs, and environmental constraints.

When planning a hydroponics garden, all these factors should be considered before deciding to purchase artificial substrates in bulk. To illustrate, organic soils are attractive in a logistical sense because the same soil can be reused each growing season. However, when it comes to accuracy with fertigation programs, soils inhibit accurate feeding schedules. Because, they are often heavily amended with fertilizer products. Conversely, artificial substrates generally don’t perform well outdoors in hot, arid conditions. They don’t retain water well enough to support plant growth in such extreme climates.

Artificial Substrates and Water Retention in Hydroponics Cultivation

For hydroponics cultivation, in cannabis and beyond, the use of artificial substrates is consistently growing in popularity. Most CEA horticulturists favor the use of these substrates because they are “inert.” This means they contain nearly no additional fertilizers or amendments. Point being, most artificial substrates allow growers exact control over what they are feeding their plants. This is an obvious extension of CEA methodology. The most popular types of artificial cultivation mediums are coco noir, Rockwool, Pro-Mix, clay pebbles, and perlite.

Most hobbyist gardeners are not aware of the fine nuances of horticulture relating to different cultivation mediums. This info is vital because, he exact same species and strain of a plant will grow differently in a slightly different substrate. This notion is important because, if a grower decides to switch their substrate they must pivot their entire feeding and watering methodology to accommodate for this infrastructural change. Along this line of thought, different artificial substrates retain water at different capacities. These irregularities must be accounted for in the developmenhydroponics cultivation, artificial substrates, aquaponics t of successful irrigation and fertigation regimes. Yet, most horticulturists don’t have the luxury of implementing “trial and error” runs to decipher how water retention in artificial mediums will affect their garden’s overall performance.

Interview with Horticulturist Doug Millar Discussing Hydroponics Cultivation, Water Retention, and Artificial Mediums

To shed some insight into the complexities of artificial substrate growing regarding water retention, we reached out to highly-accomplished aquaponics horticulturist Doug Millar. Over the past 20 years, Millar has worked in various capacities related to hydroponics cultivation. This experience includes commercial floriculture cultivation, greenhouse equipment design, aquaponics, and vertical farming.

For the last 10 years, he has studied and worked for Symbi Biological. Simbi is a research group in the San Francisco Bay area that specializes in sustainable food systems development. Millar’s latest project at Simbi Biological is a closed loop aquaponics system. In the project, insects, spirulina, and worms feed the fish, which in turn, feed the plants of the self-sustaining ecosystem. This cutting-edge research combines elements of traditional biology with those of indoor gardening technology. The aim of Millar’s project is helping solve food production issues worldwide. We are grateful to Millar for his time and expertise, here is what he had to say:

Of the forms of artificial mediums mentioned in this article, which is the most difficult to use and why?

Millar: “Each medium has its own learning curve. There is a difference between surviving and thriving, finding that sweet spot is really a matter of trial and error for most growers when using a new medium. I’d say clay pebbles are likely the most difficult simply because they often look dry on the top but hold moisture on the bottom. This issue often leads to problems with over-watering and pests. Materials handling is also a factor with pebbles, as all the other options are much lighter to lift and move.”

To again look to the list of artificial mediums, which type retains the most water?

Millar: “My initial answer is rockwool, but I would love to see a trial between the choices. While water holding capacity is important, I think a good adaptive zone and attentive irrigation strategy is more vital. Oxygen getting to the roots is crucial to healthy plants. Coco coir or rockwool are great at providing a good balance of air and water to the root zone. As with any of the medium choices, I think it comes down to choosing the right distribution systems for the crop with appropriately sized containers.”

Which type retains the least water? Would this be the best choice for growers in carefully controlled, indoor settings?hydroponics cultivation, artificial substrates, aquaponics

Millar: “It’s hard to say which of the media hold the least water due to the different shapes and particle sizes we can get nowadays. More surface area in the media typically means more water holding capacity. Container size also has a lot to do with how much water it holds. That said, in a carefully controlled environment, I tend to not want a big water holding capacity. This is simply due to how easily you can over water plants, which stresses the root zone.”

How does water retention in artificial mediums effect nutrient uptake for in hydroponics cultivation?

Millar: “Too much retention leads to waterlogged containers and root damage. Maintaining aerobic conditions is critical to a healthy root system for optimal nutrient uptake.”

Do you utilize artificial mediums with your aquaponics projects? How is your choice of medium related to water retention?

Millar: “My primary method is deep-water culture troughs with a dissolved oxygen level at 8ppm. The plants are started in small rockwool cubes then transplanted into two-inch net pots that float on rafts. I use rockwool [to start] simply because it’s a good place to sprout seeds and it holds together well. Obviously, the deep-water culture troughs have excellent water holding capacity!”

You are based in California, a State with obvious issues concerning water shortages. Looking at horticulture with a conservationist’s stance, would you say that artificial cultivation practices “waste” more water than traditional soil growing?

Millar: “In my aquaponics operation, I was using about 10% of the water it takes to grow a head of lettuce in the field. I think aquaponics may be the most efficient way to grow plants. However, a well-designed hydroponics cultivation system is probably similar. Soilless growing is a great tool for society. Water-wise farming can be utilized locally in places where traditional farming cannot, like cities, deserts, and even Mars!”

“I hope that we as a society consider all the options we have available to ensure food and water system security and equity for all people. There has been some push back about whether or not hydroponics and aquaponics can be organic. I realize field grown producers are not excited about having more competition in the organic market. However, we have a world to feed and if all organic inputs are used to grow a crop, it’s organic to me.”


As can be seen, hydroponics cultivators face a plethora of choices in deciding what sort of growing medium is right for them. For Millar and  others, many of these choices can be boiled down to water retention issues. Judging by the advice of Millar, a good starting point in understanding how water retention in artificial mediums effects plant growth is by considering both environment and container size. These two factors directly influence the performance of any substrate. Also, different plant species and cannabis strains react differently to wet and dry root zones. All things considered, it seems that successful hydroponics cultivation will have to account for all these unique operational factors in understanding overall garden performance.

This article originally appeared in the September 2018 issue of Maximum Yield Magazine.