Spending time understanding how to germinate seeds is an investment that will pay off many times over.
As with most things related to gardening, understanding why will help you make the right decisions in the future. You’ll save money by not making costly mistakes and have better success with your harvest as time goes on.
Here today we will go through the main reasons why seeds germinate or fail to germinate (leaving you with an empty pot of soil).
I have divided this list of factors into the following 4 subsections: Soil, Temperature, Water and Other.
Soil: Using the right type of potting soil mix
But here today we focus on potting soil as it is still the most widely used growing medium when home gardeners start seeds.
1. Potting soil is too hard or compact
For seeds to germinate your soil must be moist and aerated. And if your soil is too hard or compact it will be difficult for the root system to grow and develop as there will be a lack of moisture and oxygen.
It is easy to spot soil that is too compact as water will sit in puddles on top when you water from the top.
Should you choose to bottom water your pots, it will take forever for the soil to absorb enough water as the hard soil simply sits in the water like a piece of rock.
Your seeds will fail to germinate and die from lack of moisture and oxygen.
Solution: Use perlite or coarse sand to help aerate and loosen your soil if too compact.
2. Potting soil is too loose
Potting soil and potting soil mixes will often be of a very loose and airy texture when used directly from a bag.
Potting soil that is too loose will struggle to offer support for the root system while also failing to retain moisture.
And seeds will not germinate if the soil lacks moisture and dries up.
Solution: Add more moisture or potting soil to firm up your soil if too loose and airy. When filling your pots, use your fingers to gently firm the soil by pressing down on it.
Check out our video on how to assess the right firmness and structure for your soil in this video:
3. Potting soil too rich in nutrients
Seeds do not need any extra nutrients to germinate and sprout first leaves.
And when the first leaves have sprouted photosynthesis will provide the seedling with all the nutrients needed for at least the first couple of weeks.
If your soil is too nutrient rich, the first fragile roots your seeds set may be damaged or “burned” as they start to grow.
And it makes sense if you think about it. We successfully germinate seeds using rockwool grow cubes as well as LECA pellets without any added nutrients.
Solution: Use a lean potting soil or potting mix when starting seeds and do not add any nutrients until the seedling is established and ready for repotting into a larger pot.
Getting temperature right for seeds to germinate
Sure, we plant seeds outdoors in January when there is still frost and snow. And yes, the seeds do germinate.
But this does not mean that all seeds germinate in cold conditions and that heat is not important. In fact, for many seeds like ghost peppers, habanero peppers and cherry tomatoes, heat is necessary for successful germination.
4. Too hot for seeds to germinate
Your seeds want heat to germinate. But if the temperature is too high, the seeds will die.
It is all too common to place pots on a window sill on top of a radiator that is going full blast. And it is only later that we understand that it is an effective way to kill your seeds.
And yes, I have done that more than once.
Solution: Read the seed packet. If there are no instructions about temperature, assume that 18-22 degrees Celsius (64-72 Fahrenheit) is a good temperature.
5. Not hot enough for seeds to germinate
If you are starting seeds indoors early in the year you will most likely need to use an external source of heat for seeds to germinate successfully.
In zone 7, this means placing pots on heat mats or heat generating appliances when starting seeds indoors in January and February.
Some seeds may still germinate without extra heat – but it will take a lot longer. Other seeds, like for example ghost peppers, Padron peppers and Orange habaneros, will however not germinate in cool soil temperatures.
Solution: Follow instructions on the seed packet and place pots on heat mats on heat generating appliances like a refrigerator.
6. Wrong type of heat
To help your seeds germinate successfully you want warmth to come from underneath to warm the soil.
A perfect room temperature will not help if the pot is standing on a cold surface. But the opposite will work just fine. Placing a covered pot on a heat mat works great for us even when the room temperature drops at night.
Solution: Make sure to focus on the soil temperature over the room temperature. Use heat mats or heat generating appliances to ensure that you heat your pots from below.
Watering enough – but not too much
As a rule you want your soil to be moist but never soaking wet. And you should aim to water thoroughly and less frequently rather than a little bit of water several times a day.
And as a rule, a mild dry out is better than soaking wet soil.
7. Watering too much (Overwatering)
When starting seeds you want the soil to be moist. If the soil is too wet the seeds will rot and die. And for me the easiest method is to work with pre-moistened (or pre-watered soil).
After filling the pot with pre-moistened soil, I prefer to use a spray bottle and covered seed trays to keep the growing environment moist.
Top watering using a watering can risk displacing the seeds where all seeds float to the edge of the pot where there is less soil and goodness.
Bottom watering is in my opinion a better method than top watering. But with bottom watering you have to remember to let the pots drain off to avoid the soil getting too wet.
Solution: Be careful not to overwater your pots. Wet soil will lead to seeds that rot. Use pre-moistened soil, covered pots and then use a spray bottle to maintain a moist growing environment.
8. Dry outs and not watering enough
You do not want the soil to dry out as it may damage the seeds irreversibly.
For me, this happens when the cover for one reason or another fails to protect the soil from drying out. Especially when pots are placed on a heated surface.
Solution: Check pots daily and make sure that cover is in place (and not knocked off by a nosy Ragdoll).
Other reasons affecting germination rate of seeds
9. Storing seeds incorrectly
Storing seeds incorrectly will affect the germination rate of seeds.
And storing seeds can be made into a science where you can buy cold storage units and all kinds of equipment.
But as a rule you should remember to store seeds dark, dry and cool.
- Dark as in a room or compartment with no light
- Dry as in no moisture
- Cool as in around 2-5 degrees Celsius (35-40 degrees Fahrenheit)
The back of your refrigerator can be a great alternative or maybe a cool storage in a cellar or a garage.
Solution: Store seeds in a dark, cool and dry place. Aim for the best possible solution at hand. Remember that doing something is better than doing nothing and it is not about achieving perfection from day 1.
10. Bad seeds that do not germinate
Sometimes seeds are just empty husks masquerading as viable seeds.
We plant and wait and nothing happens. Pre-germinating seeds is a great way to get rid of old or bad seeds early.
If pre-germinating is not practical, plant a minimum of two seeds per cell or pot to increase your chances of successful germination.
Or see below for over-seeding.
Solution: Planting bad seeds that do not germinate happens to all of us. Pre-germinating seeds will help you get rid of duds early or simply plant more than one seed per pot.
11. Lack of patience
Seeds can take anything from days to weeks to germinate and sprout.
When we grow Thai basil we expect seeds to germinate in 7 – 10 days. Growing sage we expect germination in 2-3 weeks whereas hot peppers like the Bhut Jolokia ghost peppers have been known to take up to 5 weeks to germinate.
And sometimes we just have to be patient and wait a bit longer before giving up only to find that the seed is in fact in the process of germinating.
Solution: Read the seed packet and get yourself a general idea of the time it will take for the seed to germinate. I always wait for at least an extra week as I know that some seeds will need more time than others.
12. Seeds with low germination rate
And then you have the seeds that simply suffer from a really low germination rate even when you do everything right.
An example is white sage, where a germination rate of 20-30% is far from uncommon. Here over-seeding can help you achieve success.
Over-seeding is also a great approach when using old seeds like the seed packets left over from last season.
In the photo we are over-seeding last years left over arugula (rocket) seeds.
Solution: Make sure you sow far more seeds than the number of plants you are aiming to grow.
Bonus! When light is beneficial though not necessary
But it is not a must – for example I always plant lettuce seeds about 1 centimeter (⅓ inch) deep.