How to Plant Seedlings in Growstones for Hydroponic Growing

by Paula Costa, PhD
Growstone Consultant for R&D Applications

Start Up
Most vegetable growers start growing from seed. Other growers either start with seeds or rooted clones. Both have advantages and disadvantages, so for these growers the decision depends on personal preference, cost, growing space, crop risk and turn around time. This article will focus on starting up from seeds.

Seeds and Germination
When growing from seed, the use of certified seeds is recommended. Certification guarantees the genetic identity of the seed, i.e. seed can be traced back to the original seed developed by the breeder.  It also guarantees that the seed in the bag is true to the label. Finally, certified seeds are grown and processed to meet certain quality standards, which include purity of clean seed relative to chaff and dirt, high germination rates, and a minimum of other plant and weed seeds in the mix.

There are multiple options and ready-to-use systems for seed germination. No matter what particular system you choose, it has to follow simple basic rules for successful germination: A soft substrate with good water retention and aeration, and an ability to drain water away from the seeds keeping just enough moisture in the substrate. Ideally, the system should also allow maintaining a reasonable relative humidity of the air around the germinated seedlings and increasing the substrate temperature.

One of the best and simplest systems for successful seed germination is to use a germination tray with multiple small cells on top of a sub-surface irrigation tray. The cells on the germination tray hold the substrate where seeds will be placed and germinate (germination plugs). It needs to have drainage holes at the bottom of each cell to allow for drainage. The sub-surface irrigation tray does not have any holes, so it holds the water added during the germination and growing phase of seedlings. This tray should fit snug below and around the germination tray.

To increase temperature of the substrate, which will accelerate the germination process and seedling development, there are ‘heat mats’ that can be placed underneath the sub-surface irrigation tray. After germination, to increase relative humidity around the seedlings, you can use a ‘humidity dome’ that fits tight on top of the germination tray.

If/when the relative humidity or air temperature is too high or too low, simply adjust the openings in the dome. Both trays, ’humidity dome’ and ‘heat matt’ are available in most retail hydroponic stores and some garden centers with an hydroponic section.

You can opt for a ready to use germination system, which cells in the tray come already filled with a germination substrate, or opt to buy the parts (trays, substrate, dome, heat matt) and do it yourself.

Below are the steps to follow in the second case.

  1. Fill the cells with a soft substrate, such as a peat base mix, coco coir, foam plugs, or stone wool. These will be your germination plugs.
  2. Thoroughly soak the substrate and let drain before placing it on top of the sub-surface irrigation tray in order to avoid excess water content in the substrate. Before placing your seeds in the plugs, make sure the substrate is moist to touch, but not soaking wet.
  3. Carefully place one to two seeds in the center of each plug. If your seeds are too small (such as tomato seeds), use a wet tooth pick to grab and drop the seeds at the center of the plug.
  4. Once all plugs have seeds, cover the tray with the ‘humidity dome’, or with a thin layer of vermiculite and keep it in a dark space until germination occurs.
  5. Whenever the substrate feels dry to touch, add a thin layer of water to the sub-surface irrigation tray and watch if more water is necessary to moisten all the plugs homogenously until the substrate feels slightly moist to touch.
  6. After germination (when cotyledons emerge), place the tray under grow lights or bring to partial sun.
  7. Start irrigating with a diluted nutrient solution for seedlings.
  8. Ideally, at this point, the ‘humidity dome’ should be placed to cover the seedlings in order to keep a high relative humidity. Alternatively, you can build a clear plastic cover over the seedlings without touching the plants, for the same effect, making sure to make some holes to facilitate air exchanges. If using the ‘humidity dome’, adjust the openings to reduce/increase the relative humidity and temperature.
  9. When true leaves are visible, go over the tray and remove by hand the seedlings that look less vigorous from each plug. This will allow keeping only the most vigorous plants.
  10. Once healthy roots are visible from bottom and side of the plugs, it is time to transfer the seedlings into your hydroponic system filled with Growstones.

Planting into Growstones
Keep in mind that young roots of seedlings are very fragile and should be handled gently. Whatever your hydroponic system may be, follow these simple steps for planting in Growstones:

  1. Start by filling your growing container with Growstones halfway up. The size of the container should be proportional to the expected final size of the plant. For example, tomato plants develop a dense root system to sustain a lush and relatively large plant, thus a container size between 15 and 25 gallons is recommended to avoid root bound.
  2. Gently detach the plugs (substrate + seedling) from the tray, one plug at a time, keeping the substrate attached to the roots.
  3. Place the plugs at the center of your container above the Growstones. If the container is large and wide enough, you can place two plugs per container.
  4. Fill the rest of the container around the plug with Growstones up to 1” to 2” below the edge of the container.
  5.  Irrigate immediately after planting to promote contact between the new substrate and roots.

Note: For Ebb-Flow systems, due to Growstones light weight, it is important to use completely wet Growstones in order to keep pots from tipping sideways during initial water Ebb-flow cycles when plants are very small. This is not an issue on all other hydroponic systems.

Irrigation schedule & implications on growing
The irrigation schedule and the physical properties of the substrate are the most important factors determining how plants will develop. There are no strict recipes for irrigation when growing hydroponically. The best irrigation schedule should be determined by your crop and growing goal: Are you growing flowers and fruits, or leaves and other vegetative parts?

As a general rule, an irrigation schedule which allows for a certain dryness of the root zone between irrigations and especially during the night period and early morning/light period, results in a earlier onset of flower development, and in some cases with more flowers. This is due to the fact that water stress is a trigger for reproductive plant development. This means the plant shifts more of its energy (photoassimilates) into reproductive parts such as flowers to guarantee seed production and thus the survival of future generations. With a high irrigation frequency and root zone constantly moist, the opposite occurs, i.e. more vegetative parts, such as leaves and stems are preferentially developed compared to flowers.

The control over the moisture content of the root zone is instrumental to grow a good crop. This is achieved by the right combination between the substrate with respect to aeration and drainage, and irrigation schedule.

Most important in any growing medium is the air in the medium after drainage. Plant roots require air (particularly oxygen) for growth process respiration. However, it is important to note that plants vary widely in aeration requirements (percentage of air space after irrigation water has drained away). For example, while the aeration requirements of azaleas, ferns, and epiphytic orchids are very high; they are very low for plants like carnations, roses, geraniums, palms, strelitzia, and grass (Cornell Cooperative Extension, Container Growing Mediums and Amending Garden Soil, 2000).

An ideal substrate has small and large pores (micro pores and macro pores). When the substrate is irrigated, water is held in the micro pores but quickly drains through the macro pores, allowing air to flow through the soil, which brings oxygen to the roots and removes carbon dioxide from the root zone.

Typically these substrates are characterized by fast drainage, a reasonable reservoir of water after drainage, and most importantly, a high percentage of air spaces after drainage. They also allow for rapid changes in the concentration of the nutrient solution (EC or TDS), as it is easier to reduce/increase the concentration of nutrients available to the roots in substrates which water content can change fast.

These characteristics are present in low bulk density, highly porous substrates such as Growstones.

Thus, if the focus of growing is to produce a leafy crop (vegetative part), the most suitable irrigation schedule should be based on irrigations frequent enough to keep Growstones moist to touch at 1” to 2” below the surface between irrigations, and irrigate long enough to see some drainage at the bottom of pot (when applicable) each time you irrigate. Initially, with small plants, 2 to 3 irrigations per day could be sufficient. As plant grows and develops more and larger leaves, the irrigation frequency/duration needs to be increased/adjusted in order to accomplish the same goal and satisfy an increasing transpiration surface (larger area and number of leaves).

If the focus is to produce flowers or fruits, the irrigation schedule should allow for periods of root zone dryness. However, if a drier root zone can be beneficial in achieving the goal of more and faster flowering, it can also be dangerous if plants are not closely monitored. Make sure to observe your plants and that irrigation occurs preferentially before any visible signs of wilting.

Some plants do seem to benefit from drying a bit further and respond well to irrigation even after wilting is visible. Make sure your plant falls under this category before setting your irrigation schedule.

Irrigation Equipment
Even before planting, it is important to start thinking about irrigation. Make sure you have all the equipment necessary to control the irrigation schedule (frequency and duration), and monitor nutrient solution pH and concentration (electric conductivity, EC or Total dissolved solutes, TDS).

Unless you are irrigating continuously, you will need an irrigation timer to set up an irrigation schedule. There are multiple easy to use irrigation timers available in the market.

In order to monitor the pH and concentration of your nutrient solution (EC or TDS), you will need to purchase a reliable pH and EC or TDS meter. There are several brands available in the market today.

Hydroponic nutrients are sold as concentrated solutions and have to be added to your hydroponic system water at a certain ratio to achieve the adequate concentration. Ideally, the concentration of nutrients in solution should fall between an EC of 2.0 and 3.0 mS/cm (corresponding to a TDS around 1000 to 1500 parts per million, ppm according to NaCl conversion), and pH between 5.5 and 6.5.

Often, the meters will be able to read a wide range of values, so it is important to choose a pH, EC or TDS meter which reading range falls within the normal pH and EC or TDS range appropriate for your nutrient solution. Due to calibration and maintenance issues, separate meters for pH and EC or TDS are recommended, instead of the pH and EC or TDS combo.

You will also have to purchase the respective calibrating solutions and buffer to keep the pH and EC probes in good working conditions and get meaningful readings.

Daily – Monitor pH and EC or TDS of the nutrient solution.

As needed – Make sure the levels of the nutrient solution in the reservoir tank are kept constant or at least within range, by adding fresh water as needed for a period of time.

Every two weeks – Flush the system and make fresh nutrient solution. If there are no signs of diseases, it is a good idea to discard the old nutrient solution around your garden to take advantage of nutrients still present in the nutrient solution.

Each time you make fresh nutrient solution, let the system operate for a few minutes in order to completely mix water and nutrients before adjusting pH, as the final pH of the nutrient solution is affected by the nutrients used.

Reusing Growstones
Growstones can be used multiple times as they keep their physical integrity through time. All it requires is a vigorous shake to remove most of the roots, and placing the Growstones back in the system. As with any other media, sterilization of used media between crops is recommended. This can be accomplished either by flushing the hydroponic system and used Growstones with diluted beach solution. When available, steam sterilization is also a great option.

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7600 Reading Ave SE
Albuquerque, NM 87105