Do You Need A Humidity Dome For Seeds? (Read This First)


Every gardener wants to get a high germination rate when planting seeds.  A humidity dome is one way to help you achieve this.  However, I was wondering if it is necessary to buy a humidity dome to get this result.

So, do you need a humidity dome for seeds?  No, you do not need a humidity dome for seeds – your seeds will still germinate without it.  However, a humidity dome can help you to get higher germination rates for your seeds.  It can also cut down on the time and effort needed to keep soil moist for seed germination.

When using a humidity dome, there are some other questions to address as well.  For example, does the humidity dome need vent holes, and do you need light to germinate seeds in a humidity dome?  We’ll get to these and some other questions, but first, let’s look a little more closely at humidity domes.

Do You Need A Humidity Dome For Seeds?

As mentioned before, seeds will germinate just fine without a humidity dome.

However, it is true that a humidity dome can increase the germination rate. (The germination rate is the percentage of planted seeds that will germinate, or sprout, potentially growing into full-size plants).

This can be helpful if you are trying to germinate older seeds. For more information, check out my article on how long seeds last.

A humidity dome can also speed up the process of germinating seeds.  This is especially useful if you are growing in a commercial operation where time is money.

What Is A Humidity Dome?

A humidity dome is a cover, often made of plastic, used on top of a seed flat (tray) or other container that holds the growing medium, such as soil.  The purpose of a humidity dome is trap moisture in the air underneath the cover.

This increases the relative humidity under the dome.  This, in turn, helps to keep the growing medium moist, and encourages seed germination.

Ideally, the relative humidity will be at 98% to germinate seeds, which is much easier to achieve under a humidity dome. If you need to increase the humidity, check out my article on the best humidity level for plants (and how to achieve it).

If you wish, you can make your own humidity dome – more on this later.  However, you can also buy humidity domes in many shapes and sizes.

Many domes will fit a standard 10” by 20” plant propagation tray.  If you also want to propagate plants from cuttings, choose a taller tray.

seed trays
You can buy humidity domes to fit a standard 10 inch by 20 inch seed tray.

You can find some domes that are 7.5 inches high or taller.  For more information, check out humidity domes on the Bootstrap Farmer website.

Many domes come with adjustable humidity vents.  These vents are helpful in controlling moisture levels under the dome.  This can be a real life-saver (or plant saver) when you want to help your seedlings to acclimate to the environment outside of the dome.

Does A Humidity Dome Need Holes?

Strictly speaking, a humidity dome does not need holes.  The purpose of the dome is to trap moisture in the air underneath the cover, to increase humidity.

In some cases, vent holes in a dome may be helpful if high humidity is a problem.  For instance, high humidity levels over extended periods of time can lead to mold growth in soil or on seedlings.  This makes vent holes in a dome more attractive if you are planting seeds that take a longer time to germinate.

Vent holes are often found on the top or sides of a humidity dome, but you can put them anywhere you wish if you make a custom dome.

Achieving the proper humidity levels is a balancing act.  Too much humidity can result in moldy soil or seedlings, as mentioned above.  Too little humidity can result in the soil drying out, killing your seeds and reducing germination rates.

squash seeds
Seeds may become moldy if humidity is too high for too long, but this is more likely to affect seedlings than seeds.

Do You Need Light To Germinate Seeds In A Humidity Dome?

No, you do not need light to germinate seeds in a humidity dome.  Seeds use gravity to determine which way to grow, so they do not need light as a cue to help them.  In fact, many seeds sprout in complete darkness, buried far underground.

However, the story changes after your seeds germinate.  Seedlings do need light to grow, and they will find it in any way they can.

If there is a low level of light, the seedlings will “stretch” themselves out as they grow towards the light.  Over time, the seedlings will become “leggy”, or long and spindly, as they continue to stretch themselves out to get closer to a weak light source.

To prevent seedlings from becoming long and leggy, provide grow lights after the seedlings germinate, or put the seedlings in an area with some sunlight.  Just make sure to harden the seedlings off properly.

LED lights
An LED that provides white light is a good choice for growing seedlings indoors after they germinate.

Hardening off means is that you gradually change the light, temperature, watering, and humidity conditions of the seedlings.  That way, there is less chance that you will kill or damage the seedlings due to the shock of a rapid change in environmental conditions.

For more information, check out this article on seeds and seedlings from the Penn State University Extension.

When Do You Take The Humidity Dome Off Of Seedlings?

You should remove the humidity dome after the seeds germinate and turn into small seedlings.  The time will vary depending on the plant, but it will range from a few days to a few weeks.

If you leave the dome on too long, you risk creating conditions where mold and other fungi can grow.  This puts your entire seed tray at risk, so be sure to monitor the seeds every day and pull off the dome when the time comes.

radish seedlings
Plant the same type of seeds in one humidity dome, so that the seeds all germinate around the same time.

If your seeds germinate at different times, you can try using the vent holes on the humidity dome to adjust the humidity level.  However, it is better to avoid this problem entirely, if possible.

One way to do this is to put all of the same type of seed under dome.  That way, most of the seeds will germinate at around the same time. 

As mentioned earlier, you should “harden off” your seedlings after the dome comes off.  This means that you prepare them for the garden and the outside world by gradually getting exposing them to their new conditions.

How To Make A Humidity Dome

If you don’t want to buy a humidity dome, you can certainly save on costs by making your own.  In addition to the cost savings, there is the benefit of being able to customize your dome to the size you want.  You also get to recycle some materials from around the house.

One simple way to make a humidity dome is to fill a pot or other container with soil.  Then, plant your seeds at the appropriate depth and spacing, and water the soil so that it is moist (but not soaked).

Remember that plastic pots will retain more water in the soil than clay pots. For more information, check out my article on clay versus plastic pots.

clay pots
Filling a pot with soil, planting your seeds, and covering the soil with plastic wrap is one easy way to create your own humidity dome.

Then, use a sheet of plastic wrap to cover the pot, so that the entire surface of the soil is underneath the plastic.  You can use a long piece of plastic wrap and tuck the ends underneath the pot itself to weigh it down.  You can also just use an elastic band or piece of twine to tie the plastic wrap in place around the pot.

If you want to vent this system, all you need to do is untie the twine or elastic and pull up the plastic to let the air breathe and dry out a bit.

Another method that avoids pots, containers, and soil involves a damp paper towel and a plastic bag.  First, wet the paper towel so that it is damp, but not soaked.  The damp paper towel will be the growing medium, replacing soil in this system.

paper towel
A moist paper towel will replace soil as the growing medium in this system.

Then, put your seeds on the damp paper towel, and wrap the paper towel around them.  Finally, put the damp paper towel with seeds inside the plastic bag, and seal it.

As water from the paper towel evaporates, the humidity will be trapped inside the bag, increasing the relative humidity inside.  This will provide nice, moist conditions for your seeds to germinate.  Even better is that you can avoid the mess of soil and plant the sprouted seeds directly in the garden!

Another method that avoids pots and containers involves an egg carton and plastic wrap.  Simply put a little soil in each nook of an empty egg carton.  Then, plant a seed in the soil in each nook.

egg carton
An egg carton can also provide a good basis for a simple humidity dome system.

Water the soil in each nook, and then use the plastic wrap to cover the entire system (you can also use a plastic bag that seals, such as a larger freezer bag).  As with the other systems, the humidity will be trapped in the bag/plastic, and the seeds will germinate.

If you are handy, you might be interested in making something a little more heavy-duty.  If so, check out this article from instructables.com on how to make your own humidity dome for seeds.

Conclusion

Now you know that you don’t really need a humidity dome to germinate your seeds.  However, a humidity dome is a good way to get higher germination rates and speed up the process at the same time.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions or advice about humidity domes for seeds, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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