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


If you are starting your own plants from seed, a humidity dome may be on your mind. A humidity dome will help to get a higher germination rate to avoid wasting seeds, but it costs money to buy one or time to build one.

So, do you need a humidity dome for seeds?  You do not need a humidity dome for seeds – they will still germinate without it.  However, a humidity dome will help you to get higher germination rates for your seeds.  It will also reduce the time and effort you need to keep soil moist for seed germination.

Of course, there are other questions to consider if you decide to use a humidity dome. For example, does the humidity dome need vent holes, and do you need light to germinate seeds in a humidity dome?

In this article, we’ll take a look at humidity domes and answer some common questions about them. We’ll also get into some ways you can make your own humidity dome.

Let’s begin.

Do You Need A Humidity Dome For Seeds?

You do not need a humidity dome for seeds. Your seeds will germinate just fine without a humidity dome.

seedlings in seed tray
You do not need a humidity dome for seeds, but it can make germination easier.

However, a humidity dome has several benefits when starting seeds, including:

  • Faster Germination – a humidity dome can also act as a mini greenhouse, trapping heat in the air and soil underneath. Some seeds (such as tomatoes and peppers) germinate faster at warmer soil temperature.
  • Seed Protection – whether you start seeds indoors or in a greenhouse, a humidity dome will protect seeds from pests and diseases that spread on the wind.
  • Higher Germination Rate – a humidity dome keeps the soil warm and moist, which gives seeds the ideal conditions to sprout. This gives you a higher germination rate, which reduces wasted seeds.

A higher germination rate is helpful if you are trying to germinate older seeds.

For more information, check out my article on how long seeds last.

What Is A Humidity Dome?

A humidity dome is a cover used on top of a container that holds the growing medium (soil) and seeds. A humidity dome may be made of plastic or glass.

water bottles
You can make your own humidity dome from plastic bottles, plastic wrap, trays, pots, and food containers.

The dome is usually clear, since this will let in sunlight for newly sprouted seedlings. A humidity dome also traps heat in the air and soil underneath, acting as a sort of mini greenhouse.

However, the main purpose of a humidity dome is trap moisture in the air and soil underneath the cover.

This trapped moisture increases the relative humidity under the dome.  This keeps the growing medium moist, which encourages seed germination.

Ideally, the relative humidity will be at 98% to germinate seeds. This humidity level 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).  You can also buy humidity domes in many shapes and sizes, including:

  • Round Humidity Dome – also called cloches, there are often used to protect a single plant from cold and pests. However, you could also use a cloche as a humidity dome, such as this one from Gardener’s Supply Company.
  • Square Humidity Dome – you can also find square humidity domes, such as this 5 inch by 5 inch one from Bootstrap Farmer.
  • Rectangular Humidity Dome – this is the most common type, since they fit perfectly over many common seed trays.

Many domes will fit a standard size 10” by 20” plant propagation tray (seed 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 humidity domes that are 7.5 inches high or taller.  For more information, check out humidity domes on the Bootstrap Farmer website.

Many humidity domes come with adjustable humidity vents to let in air or let out water.  These vents make it easy to control moisture levels under the dome.

This is a real life-saver (or plant saver) when you want to help your seedlings to acclimate to the environment outside of the dome. It also helps to prevent seedlings from getting too hot under 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 and increase humidity.

In some cases, vent holes in a dome are 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.

Mold or fungus growth on seedlings is called damping off. You can learn more about it in my article here.

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

When To Open Vents On A Humidity Dome

You should open the vents on a humidity dome if the temperature under the dome is too high for your seeds.

thermometer
If the temperature under the dome gets too hot, seeds may germinate slowly or die.

You can find the ideal soil temperature for seed germination for many common vegetables in these tables from the University of California.

You will also want to open the vents on a humidity dome (or remove it completely) if some of the seeds have sprouted.

According to the Bootstrap Farmer, mold is much more likely if you leave the humidity dome on past the germination phase (the time it takes seeds to sprout).

If there are unsprouted seeds, use a spray bottle (plant mister) to give them a precision dose of water to keep the soil moist after removing the dome.

Vent holes are often found on the top or sides of a humidity dome. You can put the vent holes anywhere you want if you make a custom dome.

As with many things in gardening, achieving the proper humidity level for seed germination is a balancing act.

Too much humidity will result in moldy soil or seedlings, as mentioned above.

Too little humidity will result in the soil drying out, which will kill some of your seeds and reduce germination rate.

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

For most seeds, you do not need light for them germinate seeds in a humidity dome.  According to Science Focus, seeds use gravity to determine which way to grow.

pumpkins seeds
Seeds use gravity to determine which way is up. Most seeds do not need light to germinate.

As a result, most do not need light as a cue to help them germinate.  In fact, many seeds sprout in complete darkness, buried far underground.

Of course, there are some seeds that need light (or a thin layer of soil) to germinate. You can read more about them in my article here.

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 not enough 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. Another option is to put the seedlings in an area with enough sunlight (such as a bright windowsill).

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

Just make sure to harden the seedlings off properly before your transplant them outside. 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 To Remove The Humidity Dome For Seedlings

According to the University of New Hampshire, you should remove the humidity dome after the first leaves appear on the 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 humidity dome on for too long, you risk creating conditions where mold and other fungi can grow.  This puts your entire seed tray at risk.

Monitor the seeds every day and watch for growth. Remove the humidity dome when the seeds start to sprout.

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.

One way to do this is to plant only seeds of the same type under a 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 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 and shape you want.

As an added bonus, you also get to recycle some materials from around the house. There are a few different ways to make your own humidity dome:

  • Container and Plastic Wrap
  • Paper Towel and Plastic Bag
  • Egg Carton and Plastic Wrap
  • Tray and Plastic Bottle
  • Food Container and Lid

Each method has its own advantages, and the best one really depends on the materials you have available.

Humidity Dome From A Container & Plastic Wrap

One simple way to make a humidity dome is to fill a pot or other container with soil. It is even better if the container has drainage holes, since this will avoid soggy soil for your seeds and seedlings.

Next, plant your seeds at the appropriate depth and spacing. Water the soil so that it is moist, but not soaked.

Then, use a sheet of plastic wrap to cover the pot. Wrap the edges of the plastic around the top of the container so the entire surface of the soil is under the plastic.

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.

If you wish, 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.

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.

Humidity Dome From A Paper Towel & Plastic Bag

Here is another way to make a humidity dome that avoids pots, containers, and soil. All you need is 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.

Next, put your seeds on the damp paper towel, and wrap the paper towel around them.

Then, put the damp paper towel with seeds inside the plastic bag. Seal the bag to keep the moisture inside..

As water from the paper towel evaporates, the humidity will be trapped in the air inside the bag. This will increase the relative humidity inside the bag.

This will provide nice, moist conditions for your seeds to germinate.  As an added benefit, you can avoid the mess of soil and plant the sprouted seeds directly in the garden or in a larger pot!

Humidity Dome From An Egg Carton & Plastic Wrap

This humidity dome setup also avoids pots and containers. It only requires an egg carton and plastic wrap.

First, put a little soil in each cell of the empty egg carton.

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

Next, plant a seed in the soil in each cell. Water the soil in each cell.

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 listed here, the humidity will be trapped in the bag/plastic, and the seeds will germinate.

The only potential drawback of this system is that wet egg cartons may disintegrate, making them difficult to move without breaking. To avoid this, put your egg carton on a tray so that you can move it easily when you need to.

Humidity Dome From A Tray & Plastic Bottle

Plastic bottles are easy to find, so if you have a small tray, then this is an easy way to make your own humidity dome.

First, put some soil in your tray. Leave space at the top for watering, or else the water will spill out.

Next, plant your seeds in the soil. Water them so the soil is moist, but not damp (a spray bottle works well for this).

Then, cut the bottom out of the plastic bottle (be careful not to cut yourself).

water jugs
Cut the bottom out of a plastic bottle and use the top as the dome for your humidity dome.

Finally, put the top of the plastic bottle over the soil where the seeds are.

You can use the cap on top of the plastic bottle as a vent for your humidity dome!

Humidity Dome From A Food Container & Lid

If you have a food container with a lid that you can spare, you can use it as a humidity dome.

First, fill the bottom of the container with soil. Leave a little space at the top for air, watering, and seedling growth..

Next, plant your seeds in the soil. Water the seeds so that the soil is moist, but not soggy.

Then, put the lid on the container. A clear lid will give seedlings more light after they sprout.

Pay close attention and remove the lid after your seeds germinate. Your seedlings will not be happy if they find their progress impeded by a lid as they grow taller!

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.

This will save you money on seeds, and it will save you time on keeping soil moist for seeds.

I hope you found this article helpful – if so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

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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