When To Transplant Rockwool Seedlings For Aquaponics

In choosing when to transplant rockwool seedlings for Aquaponics, we must consider germination medium and methods for nursing seedlings for transplant. Germinating and transplanting seedlings using rockwool has been a growing trend for hobbyists. Another commercially popular medium for germination is peat moss. First, we need to discuss the germination of seedlings in rockwool and peat moss, then how to transplant the seedlings from the rock wool.

How To Start Seeds In Rockwool and Other Mediums

Choosing Your Seed

Before deciding when to transplant rockwool seedlings, we need to start with the seeds. Plants all have their own recipe for growth, it is up to the grower to understand what the plant needs. Seed manufacturers already provide pertinent information on what their varieties require. In general, these are what we will have to consider in your plant variety to understand how to grow them;

  1. It is important to understand the plant’s water requirement. Plants that have higher water requirements will require more frequent watering.
  2. Temperature is also a major parameter in plant growth as this is a factor in triggering germination. Finding the seed that adapts to your location’s temperature will increase your rate of germination. Plants also develop cold stress or heat stress depending on their variety. Plants undergoing heat stress may also require more water to cool themselves.
  3. Sunlight is important for young seedlings as well to avoid developing plants with long and weak stems. Different plants have different photosynthetic requirements, understanding your plant’s light cycle helps in allowing plants to grow with strong stems to hold itself up.
  4. Physiological structure, namely it’s stem strength is also directly affected by the amount of wind it is exposed to. Seedlings not exposed to any wind show long and weak stems, while seedlings exposed to some wind were shorter and sturdier.
  5. Plants are most susceptible to pests and diseases, but some seeds have been cross-bred to naturally develop resistance to these. Although some plants have resistances already, it is best to ensure proper protection of plants from pests by raising them above the soil and keeping them inside nettings.

Choosing Your Medium

Just like soil, germination mediums have different properties as well. Once identifying the growing conditions your seed needs, it’s a matter of identifying what kind of medium suits it. Parameters that you would need to consider in choosing your medium for germination are;

  1. Water retention of mediums differ and will affect how much water your seedling will get. A higher water retention does not mean it’s better, some plants prefer cycling between wet and dry to allow for roots to respirate as well. Generally, porous materials like rock wool and peat moss have high water retention and will require less frequency of watering.
  2. Plant roots are not only used for taking up water and nutrients, but also for respiration. Roots will naturally cycle between wet and dry, for photosynthetic and respiratory activities respectively. Respiration of plants happens during their dark cycle when there is no photosynthesis occurring. Roots will transition from being wet roots to dry roots over the course of around 4 hours
  3. Seeds contain most of the nutrients plants need to grow initially for the first week after germination. After which, it is important to understand your growing medium’s chemical composition. All plants in this stage will be starting with vegetative growth, and two of the most important nutrients for this are nitrogen and calcium. Acidity or pH, as well as organic matter content will also affect the plant’s nutrient uptake rate. In general, you would want a substrate that is pH neutral and high in organic matter content
  4. Structure of the growing medium is also important for root development. Compact substrates like clay soil will usually cause waterlogging and make it hard for your roots to respirate.

Germinating Seeds In Rockwool and Other Mediums

Rockwool seed starting is simple. It has become commonly used as a medium for germination by a lot of home growers as well as commercial ones. Rockwool is known for its high porosity and capacity to store nutrients and water. Although, untreated rockwool has high pH due to how rockwool is made and thus requires treatment before using as a medium. Aside from the porosity, another advantage is that roots can freely grow inside and around the rockwool, allowing you to transplant without disturbing the roots.

How To Prepare and Treat Rockwool For Germination?

  1. Prepare a bucket of water and add commercially available pH down. Be sure to pick pH down for freshwater aquariums that do not contain Sodium bisulfate. Sodium will raise electric conductivity causing problems with water and nutrient uptake.
  2. Once the pH of the water is at 6, add in the rockwool to soak for 24 hours or overnight. The goal is to reach a pH between 6 to 7 as this allows for most nutrients to be readily available for nutrient uptake.
  3. Drain the water and let the water from the rockwool naturally flow out. Avoid squeezing the rockwool as this will compact the fibers making it more difficult for roots to pass.
  4. Place the rockwool in your preferred seedling tray or plastic container, you are now ready to germinate.

Germinating Seeds In Rockwool and Prepare Them For Transplant?

  1. Place a seed either on top or 2 cm deep into the rock wool. Avoid placing multiple seeds into one rockwool as this will make the plants compete for nutrients and space from each other, lowering your yields in the long run.
  2. If your germination rate is not 100%, you can reuse the rockwool that do not have germinated seeds by adding a new seed.
  3. After 1 week after germination, prepare a calcium nitrate solution with a concentration of 10000 ppm by adding 10g of calcium nitrate per liter of distilled or dechlorinated water. According to research, papaya seeds that were germinated in a solution of calcium nitrate and potassium nitrate showed an increase in germination %. Personally, I do not supply potassium until after transplant due to cost-efficiency. It is up to you to use potassium nitrate as potassium helps with root development. Nitrogen is the most important element for plant vegetative growth, and calcium supports the uptake of nitrogen as well as strengthens the stem structure of your seedling.
  4. Use this solution to water your seedlings at least once a week, while the rest of the week you can use distilled or dechlorinated water.
  5. Once leaves have fully opened, make sure your seedlings get at least 4 hours of direct sunlight every day. The sunlight requirement of your seedlings will grow over time as well depending on the plant.
  6. Continue until the seedlings are ready for transplant, the time it takes for seedlings to grow will vary among cases and varieties so be sure to understand what you are growing.

How To Germinate Seeds In Peat Moss?

Peat moss or sphagnum are dried substrates made from organic matter. Personally, this is my preferred medium for germination in commercial agriculture operations. Peat moss allows for better nutrient exchange in the roots due to the organic matter content. Commercially available peat moss from companies like Klasmann also is nutrient and pH fixed depending on the chosen product. The structure of peat moss also allows less shock from transplanting as the roots and peat moss form a plug-like structure that is easy to pull out and transplant into net cups or directly into soil or your preferred growing medium.

 germinating seeds

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How To Prepare Peat Moss For Germination?

  1. Determine how long your seedling takes to grow, this will dictate what kind of seedling tray to use
  2. For seedlings that take below 21 days before transplanting, I prefer to use 50-hole seedling trays, these include lettuce and most leafy green vegetables like mustard and pakchong. For seedlings that take 20 to 30 days to nurse before transplanting which I do for tomatoes and peppers, I use a 35-hole seedling tray.
  3. Fill the holes with peat moss to the brim.
  4. Do not apply too much pressure as this will compact the peat moss, making it harder for the roots to develop fully.
  5. Commercially available peat moss are nutrient and pH-fixed, thus they are immediately ready for germination. I have seen great results with Klasmann TS3 for leafy vegetables but other commercially available peat moss will also work depending on your plant variety.

How To Germinate and Nurse Your Seeds In Peat Moss?

  1. Poke a hole in the middle of the peat moss around 2cm or a little shorter than an inch deep.
  2. Place one seed inside the hole and cover up with the rest of the peat moss. A reminder to not apply too much pressure to avoid compacting the material.
  3. Follow the directions on watering based on the plant’s water requirements.
  4. There is no need to add nutrient solution for peat moss products that already have nutrients, be careful of adding too much nutrients as well because this will raise electric conductivity, making it more inefficient for the plants to take up nutrients.
  5. As the roots develop, they will start to fill the space in between the peat moss. Once ready for transplant, the seedling’s root system will come out as a full plug wherein you can see the peat moss being held together by the root system growing around it.

When To Transplant Rockwool Seedlings and Peatmoss Seedlings

Rockwool and peat moss are great for transplanting because they allow the root system to fully develop within the space inside, making it easy for transplant. To transplant seedlings, take the seedling along with the rockwool or the peat moss and place in your preferred growing medium or net cup. Here are some tips to transplant rockwool seedlings to ensure the proper growth and development of your plant after transplant;

  1. Avoid drowning the root system, this can be done by making sure that only the tip of the net cup or the root system touches the water. The reason for this could be seen as to how the Kratky Method works well; as the plant takes up water and nutrients, the root system will naturally grow deeper and look for more water.
  2. Make sure that the plant is steady in place to avoid them falling down. In a net cup that’s bigger than the root system with the rock wool or peat moss, I add small stones to support the stem. In a media bed, have your media support the stem to keep it upright.
  3. Do not disturb the root system by pulling part of the peat moss or rock wool from it. At this point, the roots have fully developed around the germination medium so it is best to leave the germination medium with the plant as it grows.

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Different practitioners will have their own preference in mediums and practices. Seed germination requires a lot of trial and error before finding what works for you. What is most important is to understand the knowledge and how to apply this knowledge.

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