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Do cannabis seeds need light to sprout

How We Germinate Seeds Indoors

We often get beginning growers in the store looking for seed starting advice. To germinate seeds, or take them from dormancy to sprouting, you need to set up the environmental triggers to assist the seed. There are many different techniques for germinating seeds, and a lot of products and advice out there for how to get started if you’ve never done it. So much advice, that it can be very confusing. The following is a little advice for what we have found that works for a wide variety of seeds for flowers, vegetables, herbs, and even cuttings/clones:


When is the best time to start your seeds? If you are going to be growing your plants indoors, then anytime you’re ready is the best time. If you will be transplanting to an outdoor space, then it depends on the weather where you live. Check with The Farmer’s Almanac, or the back of the seed package will usually tell you how many weeks before the expected last frost date for your area to plant. Don’t know the last frost date for your region? This is a good page to help you figure that out.

Getting Your Seeds Planted

Always wear a mask when working with perlite!

Always wear a mask when working with perlite! Perlite is a form of obsidian characterized by spherlulites formed by cracking of the volcanic glass during cooling, used as insulation or in plant growth media. It is like tiny shards of glass and is very dangerous to your lungs if it’s inhaled.

Step one: Coco coir comes in a dehydrated brick. I hydrate the coir according to the package directions. Before doing this you would benefit from reading the notes in the Watering section of this post.

Step two: I mix perlite into the coir at a ratio of one-part perlite to two parts coir. The coir in our mixture holds moisture without staying too wet. The perlite helps aerate and loosen the media for water to drain easily while also retaining moisture. To keep the dust down, we recommend using water from a spray bottle to wet the perlite prior to handling. I stress again to wear your dust mask when handling perlite.

Step three: I have found that using a product with mycorrhizae increases germination rate and root growth rate after the seed sprouts. I mix the package-recommended amount on whichever product I am currently using.

Step four: Fill the seed tray with media and plant your seeds! Tip: A general rule for seeds is to plant them no deeper than one to two times the diameter of the seed. For very tiny seeds (i.e. lettuce) I simply drop a few seeds on the surface of the media and leave them uncovered.

Seeds respond to warmth and need heat to germinate. If your media, or the water you’re using, is too cold, the seeds will stay in their dormant state. Most cool weather plants or plants that do best in spring and autumn (i.e. spinach, kale, etc.) generally need temperatures between 45°F and 70°F to germinate. Seeds that require warmer weather (i.e. tomatoes and zucchini) will germinate better generally between 65°F and 90°F. Most seeds have about a 25-degree range in which germination will be activated in the seed. This is great for us beginners trying out lots of plants for the yard. If your room is cool, and you’re trying to germinate seeds, pick up an agriculture heat mat and thermostat from your local hydroponics store to dial in the ideal temperature.


Our seedling/clone rack at the Lush Lighting Hydroponic Store

The light requirement of seeds is a tricky subject. Some seeds do not require light to germinate (mushrooms), some do. There is a ton of scientific research available about this if you’re interested in learning about the phytochrome system in seeds. If you would rather just grow the plant what do you do? My advice: Use a light. Here’s why: All sprouts need light. As soon as that sprout emerges from the media it needs light; even if it didn’t need light to germinate. A seed that needs darkness, planted at the right depth, will be far enough under your media and will be shaded from the light for germination. In the store, we use a Lush Lighting Herbal Vador LED grow light hung beneath a shelf. Others have used CFL’s which are widely available (we also have CFL’s available in the store). Some others have had great success placing their seed tray in front of a south-facing window.

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A few notes about water:

  • Seeds like to be kept moist, and they don’t like to be immersed in water for too long. If seeds stay too wet for too long they will rot. If seeds stay too dry for too long they will dry out and die.
  • Doing a pH test on your water is a really good idea. When water pH is too high or too low, your seeds will not germinate. Most seeds germinate between pH levels 5.8 and 7.0. Matt Johnson wrote an excellent explanation of the importance of checking water pH.
  • Water quality can make a big difference in your success rate. If you live with city water consider using RO filtered, or bottled distilled water (READ LABELS! You might be really surprised at some of the stuff that is in bottled water being sold on store shelves!). City water has been treated with all kinds of chemicals to remove bacteria and the like. Some of those chemicals (i.e. chlorine) are not good for plants.
  • Water temperature matters. Cold water from a tap is often too cold for germination. Use water within the best temperature range for your plants. Leaving your water to sit out overnight to come to room temperature before you water your seeds is a good idea (Matt Johnson’s article –mentioned above—can give you more good reasons). The water we use to water all the plants here at the store has come to room temperature before it goes on the plants, including the seeds.

How often do we water? Water as often as it takes to keep the media moist and not drenched. We use a variety of techniques to keep our seeds happy: a mister, bottom-watering container, and/or a humidity dome.

The mister has a gentle mist that does not “skate” the tiny seeds (like lettuce) across the top of the media. It’s very important to make sure your seeds stay where you initially place them.

Another watering technique we use is bottom-watering. Put about ¼” of water in the bottom tray for your media to wick up. This technique is very helpful if your media is drying out too fast. If your growing room is dry, bottom-watering combined with a humidity dome may be a good option for you. If you need to step away from watering for a few days, it will retain the moisture for the seeds and keep any small sprouts from drying out. Small plants generally do well with a humidity level around 60%.

Be patient! Sprouts will come in due time!

How Long Will It Take before I see something happening?

A common question with an answer most of us don’t want to hear… it depends. I will say, the highest success rates come from recreating each particular seed’s ideal germination environment. Most seed packages will give you a time range anywhere from a few days to a few weeks. All seeds are a little different, even the same packet of seeds can have some that sprout quicker than others. Be patient, monitor your temperature and moisture and then you will be rewarded greatly when those first seeds germinate and those little sprouts emerge. It’s exciting to watch those small seeds become large plants. Happy Gardening!

About Matt Johnson

Matt is the President and Founder of Lush Lighting, Inc. He is a leader, a visionary, a mentor, and caring family man. Matt openly shares his wisdom, generosity and assistance with everyone he encounters. He is a sharp business man, admirable, loyal, and willing to step out of the box to make things happen. Whenever he gets a chance, he squeezes in a round of golf or a movie with his wife.

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If you want to check more on different techniques for germinating seeds, we can share to you our hidden steps.

Top 5 Germination Mistakes To Avoid When Growing Weed

Germinating cannabis seeds is necessary in order to sprout seedlings that develop into mature, healthy cannabis plants. However, complications with light, humidity, heat, and more could result in cannabis seeds failing to sprout. Find out what to avoid to ensure successful germination.

Germination is where the magic begins. Cannabis seeds must first germinate in order to sprout and begin their journey as living, breathing plants. However, if the germination process is done incorrectly, or is thwarted by some other variable, seeds can fail to sprout, leaving you with useless, spent seeds instead. Here are the top five mistakes to avoid when germinating cannabis seeds.


One of the most common reasons seeds fail to germinate is because they are simply duds. Typically, healthy cannabis seeds should look a specific way and be of a specific colour. Viable seeds will appear round, not flat, and should be a beige to dark brown colour with subtle tiger striping. Seeds that have been flattened or are pale in colour may have a tough time sprouting into healthy cannabis plants.

Seeds that have been improperly stored may also fail to germinate. These tiny packages of DNA prefer to be kept in a cool, dark place with moderate humidity. Exposing them to heat, light, or extreme humidity levels (both high and low) could result in seeds losing their viability. As long as you source your seeds from reputable seedbanks and store them appropriately, you shouldn’t have to worry about bad seeds ruining your chances of successful germination.


Generally speaking, seeds require a dark environment in order to germinate. After all, in nature cannabis seeds find their home in the dark embrace of soil. It can be hard to determine exactly how much light is too much for your seeds; however, err on the side of caution and germinate out of direct light. There’s no need to use your grow lights until after germination has occurred.

It is also likely that the quality of your seeds will affect their ability to handle direct light. Seeds that are already having a tough time germinating will have an even worse time doing so if they are exposed to excess light.


The amount of water you supply your seeds during the germination process will also affect their ability to successfully germinate. Some growers attempt to germinate by “drowning” them in a glass of water. While this ensures they will not go thirsty, it can actually be more harmful than helpful.

Once they pop, seedlings are very delicate and must be watered carefully. When germinating, your medium should be damp, but not overly wet for best results. On the other hand, a dry environment is basically a death sentence for your seeds. They need a constant source of moisture to sprout, otherwise they’re good as duds.


Cannabis seeds require a specific temperature range in order to germinate. Regardless of the germination medium you are using (soil, paper towels, etc.), the temperature in the environment should consistently sit in the 26–28℃ range. Seeds need to be kept warm during the germination process. At no point during germination should seeds be exposed to temperatures below 20℃.

There are some techniques that can be used to ensure your seedlings remain in this temperature range. Some growers choose to use special warming mats that can be set to a specific temperature and placed underneath the seedling containers. Another strategy is to place a timed heater close to your seedlings to warm them up occasionally. Once your seeds have sprouted, they will be much more resilient to fluctuations in temperature.

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Many people choose to germinate their seeds using paper towels. However, others choose to do so directly in soil. Of course, germinating your seeds in soil isn’t inherently bad, but it can come with its own set of challenges, and is generally not recommended unless necessary.

For instance, the upper layer of soil can dry out within 48 hours, making it more difficult to give your seeds the right amount of water they need without overdoing it. Giving your soil too much water during germination could result in the seed rising up or dropping down further, inhibiting its viability. Moreover, seeds that are planted too deep in the soil could experience a host of complications. They could suffocate before coming into contact with enough oxygen, and be unable to access sufficient light to progress into the seedling stage of their life cycle.

Seeds may also be unable to germinate if the soil they are planted in contains contaminants. Mould and pests can easily kill a mature, healthy weed plant within just a matter of weeks. It should come as no surprise then that they could also prevent a small seed from germinating. If your soil contains traces of these contaminants, it is possible that your seed will never actually open and sprout. This also applies to fertilisers. Even small amounts of fertiliser in your soil can effectively kill your seed, making it completely useless.

When can seedlings be put under lights or in the sun?

Not sure whether to grow your cannabis seeds in the sun or under lights? This article explains how much light you need, how to prevent your seedlings from falling over, and when to plant them outside.

“When can cannabis seedlings be put under lights or in the sun?” is a common and sensible question that is often asked by novice cannabis enthusiasts after germinating seeds indoors under lights.

For those who are growing indoors, cannabis seedlings may be put under lights as soon as they emerge from the soil or growing medium.

How much light for cannabis seedlings?

When using HID lighting (usually a metal halide lamp), young seedlings should be kept at least 50cm from the bulb.

With compact fluorescent lamps (usually 100w or more), a distance of around 15cm should be maintained between the top of the young seedlings and the bulb.

If using normal fluorescent tubes (18-36w), seedlings can be kept within a few centimetres of the light source.

Cannabis seedlings growing under lights should always be given a gentle breeze from an oscillating fan (a small household fan on the lowest setting is fine), as constant gentle movement will strengthen their stems significantly.

Are your cannabis seedlings falling over?

If cannabis seedlings grow tall and then fall over, this is almost always a result of growing in an environment with static air.

Seedlings intended for outdoor growing should be kept by a sunny window for the first week or two after emerging from the soil.

When they have grown their second or third set of serrated leaves (after the the round cotyledons that initially emerge from the seed) seedlings are usually hardy enough to flourish in direct sunlight.

When to plant your cannabis seedlings outside?

If outdoor temperatures are suitable, seedlings may be acclimatised to direct sunlight by giving them progressively longer daily exposure to outdoor conditions.

Starting with about three hours outside, at the sunniest time of day, seedlings can be given an extra hour of outside exposure each day, so that within about two weeks they can be left outdoors permanently.

Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.