Germination is the process of a seed transforming into a young plant. To achieve successful seed germination, the seed requires the right conditions. If you’re holding a pack of dry seeds, they will remain dormant unless given proper care. Seeds must be placed in a moist growing medium, such as soil, rockwool, or even a wet paper towel. While seeds aren’t too particular about the type of medium, they are very picky about moisture. The growing medium should remain moist 24 hours a day for optimal results.
At the ideal moisture and temperature levels, germination begins. Water is absorbed by the seed, initiating rehydration. This triggers metabolic activity, which leads to cell division. Soon, roots form and anchor the seedling, culminating in the emergence of a young plant.
Technically, a seed is a plant embryo and a living organism in a dormant state. Germination is the process that awakens the seed from dormancy, driven by environmental factors like moisture, temperature, and oxygen. During hydroponic seed germination or soil-based methods, hydration is the key step. Once rehydrated, seeds resume metabolic functions, leading to growth.
Most seeds germinate best at temperatures between 65–80°F (18–27°C). However, some plants like lettuce can germinate in colder temperatures as low as 40°F (nearly freezing). While germination at sub-optimal temperatures is possible, it’s much slower. To achieve the best results and fastest germination times, maintaining consistent, optimal temperatures is crucial.
For hydroponic seeds, consistent moisture and warmth are particularly important. This is why many growers choose controlled environments to ensure the best germination for hydroponics.
Germination times also vary depending on what you’re growing. Here is a summary of a few different plant types, growing at an optimum temperature:
| Type | Germination Time | Type | Germination Time |
|---|---|---|---|
| Basil | 7 to 14 days | Lettuce | 5 to 14 days |
| Chard | 5 to 17 days | Parsley | 10 to 14 days |
| Chives | 7 to 14 days | Squash | 7 to 14 days |
| Cilantro | 14 to 21 days | Spinach | 10 to 14 days |
| Kale | 3 to 10 days | Tomato | 10 to 14 days |
Why You Should Learn to Germinate
Learning how to germinate is a very useful skill to know. Especially if you have a garden! It is useful for a few reasons:
• You are more autonomous. You have more power and flexibility in what you can grow.
• Seedling purchases and subscriptions are over-priced.
• Live seedlings often arrive damaged, stunted, or not alive. This wastes time.
• Seeds can germinate in the same time it takes to receive a shipment.
Germinating your own seeds will save a lot of money in the long run. Once you learn to germinate, it is a skill you'll have for the rest of your life. There's also a great sense of satisfaction in planting a seed and watching it grow. Watching your seedling grow into a mature plant and harvesting it. It is empowering.
The autonomy you get from learning to germinate is really the best part though. You'll own the entire process from A to Z. From soup to nuts.
Plus, it’s easy! We’ll show you how.
Rockwool
Germination can take place in different types of growing mediums. Of course – soil is a medium we all know well. But with hydroponic systems we don’t use soil. So what shall we use?
By far – the most popular hydroponic growing medium is rockwool. Rockwool cubes to be more precise. They come in sheets with the cubes scored which makes them easy to pull apart. The cube shape is great because they fit better into net pots when transferring seedlings to your tower.
When they come in sheets, cube sizes vary from 3/4“ to 2”. We recommend you use the biggest size cube that can fit into your net pot. Rockwool is a bit malleable so even if it’s slightly bigger than your net pot, you can still stuff it in. Bigger cubes sit more flush against the sides of the net pot, making them less wobbly and more secure. This provides a more sturdy base for your plant to grow.
Rockwool is made of molten rock that has been spun into fine fibers. These fibers are what gives rockwool its wool texture. Due to the high heat involved in the manufacturing process, rockwool provides a bacteria-free, sterile medium in which to grow your plants.
In addition, rockwool has certain properties that make it great to use for germination:
Water retention – rockwool fibers are great at retaining water. They don’t dry out easily and hold water for a long time.
Porous nature – at the same time, rockwool fibers are porous so they don’t hold too much water either. It is really hard to under-water or over-water rockwool making it somewhat foolproof for newbies. Additionally, the porous nature of rockwool provides adequate space for roots to grow.
About sponges: sponge cubes can also be used for germination. But they dry out faster than rockwool so keep a careful eye on your moisture levels. Additionally, when planting sponge-based plants in your tower - you should leave your watering cycle to run 24 hours a day, instead of 10 - 14 hours a day (with rockwool).
How to Germinate in Rockwool
Once you learn how to germinate seeds in rockwool you'll have a long-term skill to sustainably grow produce at home. It's easy to do and quite easy to learn. It's also really satisfying nurturing a seed and watching it grow, all the way to harvest. Before starting the germination process, you should decide how many plants you want to grow. One rockwool cube will hold one plant. Also, you’ll want to move sprouted cubes under lights (step 6 below) so have a grow light ready to go.
Step 1: Soak rockwool and drain. Take a sheet of rockwool and place it in your tray. Don't separate the cubes yet, just place the whole sheet in the tray. Use de-chlorinated water, with a pH between 5.5 – 6.5 (distilled water is safe because it has no chlorine, but you should still check the pH). Pour in water until the water level reaches about three-fourths (3/4) of the way up the rockwool. Soak this way for 10 minutes. After 10 minutes, remove the rockwool sheet and place in your kitchen sink and let drain. Just leave it there at the bottom of the sink and it will drain by itself. Let drain for 10 minutes.
Step 2: Separate the cubes. After your rockwool has drained, place the sheet back in your tray. Using your hands, separate the cubes along the scored lines. You’ll find the cubes separate very easily after they’ve been soaked and drained.
Step 3: Plant seeds. Once you've decided what you want to grow, we recommend you place two seeds into each cube. Rockwool typically comes with a hole poked through the top where you can insert the seeds. If your rockwool cubes didn’t come with holes, take a small stick and poke your own holes about 1/2" deep.
Position each seed on opposite sides of the hole, about 1/4" down. Push the seed gently into the walls of the hole. For smaller seeds like lettuce, you can use a toothpick. Dip the tip of the toothpick in water, then use it to pick up the seed. The moisture will help the seed stick to the end of the toothpick.
Once your seeds are planted, spray the top of the hole with a spritz or two of water from a spray bottle.
Note: Placing two seeds into each cube increases the chance of success, in case one of these seeds doesn't germinate. This is for redundancy reasons. Most of the time though - you'll find both seeds will sprout. Once they're about an inch tall you can remove the weaker seedling (see Step 7 below).
Step 4: Sprinkle vermiculite into the planting holes and on top of the cubes. Vermiculite has excellent water retention properties. By sprinkling vermiculite into the planting holes, this will help keep the planting holes moist and prevent them from drying out. Do not push the vermiculite down into the holes; you just want to sprinkle it in. If you push it down into the planting holes, the vermiculite can become too compact which would make it hard for the seedlings to sprout through the top of the rockwool cube. Once you sprinkle the vermiculite, give each cube another misting with your spray bottle to moisten the vermiculite.
Step 5: Maintain moisture and temperature, wait for seeds to sprout. Check your cubes daily to maintain moisture levels. Pour water (PH 5.5 – 6.5) into your tray until there’s about 1/4" of water at the bottom of the tray. Rockwool wicks up water from the bottom of the tray so you don’t need more that 1/4" at the bottom. The water will evaporate eventually, and you can refill 1/4" every couple days. Misting your cubes daily with a spray bottle also helps to maintain moisture.
Maintain an appropriate temperature for the plant you are germinating.
Seeds generally germinate best between 65 - 80 degress Fahrenheit (18 - 27 degrees Celsius), but the temperature varies depending on the type of seed. Using a humidity dome with your germination tray will help keep a warmer temperature.
In the winter months, you can put a heating mat underneath your tray too. But this depends where you live. In warmer Southern California for example, you can successfully germinate in December and January without a heating mat (you should still use a humidity dome though).
Step 6. Move sprouted cubes under a grow light. The very moment you see something green starting to poke its head out of the rockwool cube, transfer the sprouted cube under a germination grow light. Seedlings are like babies starving for food. By turning on the lights you are literally feeding your seedlings through photosynthesis – a process where plants use light to synthesize food.
Lighting placement - you want the light to be close, about 2 – 3 inches away from the top of the seedling. Basically – put the light as close as you can without burning the plant.
Lighting cycle - seedlings need about 12 – 16 hours of light per day. Continue with this lighting schedule until they’re ready to be transferred into your hydroponic system.
Keep un-sprouted cubes under the humidity dome (and on top of the heating mat if you're using one). As they sprout, move each sprouted cube underneath the grow light immediately.
Humidity domes and heating mats should no longer be used with sprouted cubes. Check water levels of both sprouted and un-sprouted cubes daily, and continue with daily misting.
What happens if you're late with the lights? In an effort to survive - their stems will stretch out as they reach for the light and they will become too tall. This is known as becoming “leggy.” More on this in the “Leggy Seedlings” section below.
Step 7. Remove second seedling from each cube. Remember we planted two seeds in each cube for redundancy reasons. More often than not – both seeds will sprout. Once they’ve grown a bit, around an inch tall - remove the weaker seedling by gently pulling on it with a pair of tweezers. Gently grab and pull and the whole seedling will come out, roots and all. Leave the stronger seedling.
Leggy Seedlings
When seedlings get “leggy” this means their stems have gotten too long. Legginess is almost always caused by inadequate lighting, lights being too far away, or not lighting as soon as the seeds have sprouted. Freshly sprouted seedlings are starving for light and they’ll stretch out to get closer to it. Another cause of legginess is overly high temperatures. If you’re using a humidity dome or heating mat, be sure to remove them as soon as the seeds have sprouted.
If the stems stretch too far, they can become permanently damaged. The stems will get too thin and too tall to support proper growth and development. Leggy seedlings become top-heavy, and will start to lean over to one side. Growth will be stunted and the plant will not form properly.
Can leggy seedlings be saved? If you catch it quickly – perhaps. First – bring a light source close to your seedlings, about 2 – 3 inches from the top. Place the lights as close you can without burning the plant. Leave the lights on for 14 – 18 hours a day. Check if temperatures are too high… remove the humidity dome and heating mat if you’re using one. Turn on a fan and give your seedlings a gentle breeze. The breeze will help strengthen the stems, giving them a little workout.
With traditional farming, a good way to save a leggy plant is to “pot-up.” Potting up means burying the plant deeper down into the soil. The soil itself is then able to support the sides of the stems a bit, so they don’t lean over as much. With hydroponic farming however – potting up isn’t possible because there’s no soil. You may eventually find yourself at the point of no return, when your plants are too leggy and you start the germination process over again. So the best solution is prevention. As soon as you see something green popping out of the rockwool cube – throw on the lights. Remove humidity domes and heating mats. Do not wait.
Transferring Your Seedlings
Alright – your seedlings are looking healthy and you think they might be ready to be transferred into your hydroponic tower garden. This part is actually pretty exciting. It’s kind of like watching your kid graduate. Your seedlings are graduating and moving on to high school! It’s actually super easy to do as well.
There are a few ways to know when your seedlings are ready to be transferred. Look for these signs:
Four leaves have formed. This is the best sign. The first two leaves you see from the seedling are actually called cotyledons (parts of the embryo). Wait until two more leaves form underneath (four "leaves" total).
Roots emerge from the bottom of the rockwool cubes. If you see roots growing out of the bottom of the cubes, this is a good sign that your seedlings are ready for transfer.
Seedlings reach 1 – 2 inches tall. This is not a telltale sign, but more of a supporting indicator. Ideally you have the signs above to go along with this one.
Place the rockwool cubes in each net pot and push it all the way down. Be carefuly not to damage the seedling - use your index finger and push down on the rockwool, not the plant. It’s important the rockwool base sits flush against the bottom of the net pot. This ensures the roots will be watered properly. Repeat this process until all the pots in your tower have been planted.
Set Lighting and Watering Schedules
Once your seedlings have been transferred, you’re going to need to set your timers. We’ll assume you’re growing indoors and have two timers to set.
Lighting Schedule – For indoor growth, a good lighting schedule after transferring seedlings is 14 hours on, then 10 hours off. Set your timer to run this schedule daily.
Watering Schedule – It’s better to set watering intervals, instead of having a continuous flow. This allows the roots to more freely absorb oxygen directly from the air. If you germinated with rockwool cubes - a good watering interval after transferring seedlings is 15 minutes on, then 45 minutes off. Run this interval for 14 – 18 hours a day (less hours when it's cooler, more hours when it's warmer). If you germinated with sponge cubes - we recommend 15 minutes on, then 15 minutes off for 24 hours a day. Remember in the warmer months, you should increase your watering since water will evaporate more quickly.
Here are some examples of watering schedules you can use (with rockwool cubes):
| Indoor Cool | Indoor Warm | Outdoor Fall / Spring | Outdoor Summer |
|---|---|---|---|
| 15 minutes on | 15 minutes on | 15 minutes on | 15 minutes on |
| 45 minutes off | 45 minutes off | 45 minutes off | 30 minutes off |
| 14 - 15 hours a day | 17 - 18 hours a day | 18 - 20 hours a day | 20 - 24 hours a day |
Once your timers are set your water and lighting will run on auto-pilot. But you should still monitor your plants daily and adjust these schedules accordingly. For instance - since their root systems are still developing, younger plants require more water than mature plants. Mature plants can also handle longer periods in between watering. Lighting schedules are less fickle, but are more dependent on what’s being grown. For example – fruits, like tomatoes and strawberries will require more light than lettuce.
FAQ
Germination is the process by which a seed grows into a young plant under the right conditions.
Seeds need a moist growing medium and the right temperature to germinate.
To germinate seeds for hydroponics, place seeds in a moist growing medium like rockwool or a wet paper towel. Keep them at an optimal temperature between 65–80°F (18–27°C) and ensure consistent moisture. This method works well for a variety of seeds, including basil, kale, and lettuce.
It depends on the seed type and environmental factors. Most seeds sprout within 3–10 days under optimal conditions.
The germination time varies by plant type and conditions. For instance:
- Basil: 7–14 days
- Lettuce: 5–14 days
- Chard: 5–17 days
- Parsley: 10–14 days
- Kale: 3–10 days
- Spinach: 10–14 days
- Tomato: 10–14 days
Seeds need a moist growing medium and the right temperature to germinate.
Seeds like lettuce and kale can sprout in as little as 3–7 days under optimal conditions. Slower seeds, like cilantro, may take up to 21 days.
Regularly check the water levels, pH balance, and temperature. Clean the system to prevent algae buildup and ensure proper nutrient delivery during the germination phase.
