What is a Humidity Dome?

There are many ways to start seeds and each plant has different requirements for seed-starting. While direct sowing is a good option for some, indoor germination is almost always a better idea. If you’re starting your plants from seed, a humidity dome may help provide higher germination rate, avoiding the waste of seeds.

Although it’s not a requirement in starting your seeds, a humidity dome can be highly beneficial, especially in dryer climates. It traps the heat and humidity to maintain a controlled environment needed for the germination process. There are plastic humidity domes commercially available, but if you don’t want this extra cost, you can also make your own humidity dome. This even gives you the freedom to choose the materials you use and the size and shape you need. 

Before you use a humidity dome, you may have a few questions. Does the humidity dome need vents, do you need light to germinate seeds in your humidity dome, and when can you remove the humidity dome? We will answer all of these questions below.

seed tray for humidity dome

What Is A Humidity Dome?

A humidity dome is simply a cover placed on top of a pot, seed tray, or any container that holds the soil and seeds. It may be made of plastic or glass, and usually transparent to let in some light for the sprouted seeds. 

Humidity domes trap the heat in the air and soil, working like a greenhouse in a much smaller scale. The cover traps moisture and increases the relative humidity to keep the soil moisture, which encourages seed germination. Ideally, you need a relative humidity up to 98% to germinate seeds, depending on the type of plant. You can achieve the desired humidity under a humidity dome a lot easier for faster germination.

You can buy factory-produced humidity domes online or from garden supply stores. They come in different types and sizes, some with vent holes that you can open and close to regulate the humidity inside the dome. 

Alternatively, you can make your own DIY humidity dome from common materials such as plastic bottles, plastic wrap, egg trays, and food containers. 

DIY humidity dome

Do You Need A Humidity Dome?

Again, it’s not strictly necessary to use a humidity dome for seed-starting, but it’s guaranteed to make germination easier. There are several advantages that may convince you to use humidity domes, particularly in a dry climate. For one, humidity domes trap the heat and moisture during germination, the time when seeds need them the most.

Improved germination

The main purpose of a humidity dome is to hold in the moisture that helps speed up the seed germination for most plants. It can also help retain heat, more so when used together with a heat mat. A humidity dome is especially beneficial for older seeds that may have a reduced viability due to a long time in storage. In any case, using a humidity dome creates the perfect conditions for most seeds. Maintaining these conditions inside the dome ensures faster and increased germination.

Seed protection

A humidity dome is specifically made for the purpose of trapping the humidity under it, hence the name, but seed protection is a welcome bonus for growers. The plastic or glass dome creates a barrier to protect your seeds (especially large flower seeds and vegetable seeds) from birds and rodents. If you have this kind of problem, you may benefit from the protection a humidity dome can provide for your seedlings, even if it’s not particularly necessary for seed germination.

Does A Humidity Dome Need Holes?

A humidity dome does not absolutely need to have holes or vents. Remember that the purpose of the dome is to trap moisture under it and increase humidity levels for the seedlings. However, vent holes can be helpful to control the humidity levels.

In some cases, too much humidity can become a problem for the seeds. For instance, too much moisture in the dome over long periods of time can encourage mold and other microorganisms to grow in the soil or on your seedlings. 

On the other hand, too little humidity can dry out the soil, killing some of your seeds. This reduces germination rate, the exact opposite of what you’re trying to achieve with a humidity dome.

While holes in the dome are not essential, you do need to vent your humidity dome periodically, whether it’s by opening the holes on top or simply removing the cover for a short time to let some of the excess moisture out. How often and how long you must vent your humidity dome may vary depending on the plant type, its light requirements, and the materials of your humidity dome.

You should check on your humidity dome at least once a day even if it has vents at the top. Open the vents or remove the cover on top and shake off the excess water if the temperature under the dome gets too high for your seeds. 

You may do this once every day or two, probably around noon, especially on a warm, sunny day. You may also want to do this if some of the seeds have already sprouted. If you have unsprouted seeds, mist them with water to keep the soil moist before returning the cover on the container or tray.

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

In most cases, you don’t need light to germinate seeds in a humidity dome. Most seeds sprout underground in complete darkness. Depending on the plant, some may need light or only a thin layer of soil or other growing medium to germinate. But after germination, seedlings do need light to grow. In fact, they will try to find a light source however they can.

If they can’t get enough light, the seedlings grow towards any light source, stretching themselves out. When this happens, they grow to become long and leggy. To prevent this, you can put your seedlings in an area that gets enough sunlight like a south-facing window for the full sun benefit, or an east-facing window for some light morning sun. Alternatively, if you can’t provide natural light, you can strategically place growlights over the seedlings.

Do You Need Light To Germinate Seeds In A Humidity Dome

When To Remove Seedlings in a Humidity Dome

You can remove your seedlings from the humidity dome after the first leaves appear. The time you need varies greatly depending on the plant, but it typically ranges from a few days to a few weeks. Some leafy vegetables start to germinate in as short as three days.

If you leave the seedlings in the humidity dome for too long, this may create the perfect conditions for mold and other fungi to grow. This can affect the growth of your entire tray of seedlings. 

To make sure you remove the humidity at the right time, check your seeds every day and watch for the growth of leaves. Then, you can remove the humidity dome once the seeds start to sprout.

But if your seeds germinate at different times, you can vent the humidity dome to adjust the humidity inside. However, the seeds may germinate around the same time and you can avoid this problem in the first place if you plant the same seeds under one dome. 

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. There is also the added benefit of freely making the humidity dome to the size and shape that you want. You can recycle materials from your house like a plastic bag or plastic wrap, an egg carton, or an unused food container.

Pot and Plastic Wrap or Plastic Bottle

Pot and Plastic Wrap or Plastic Bottle

The simplest way to make a humidity dome is with plastic wrapped over a pot or any other container where you plan to grow your plant (so there’s no need to transplant it when it’s sprouted!). 

To do this, choose the container you will use. It must have drainage holes at the bottom to avoid soggy soil. Then, fill the pot or container with enough soil and plant your seeds. Take note of the proper depth and spacing between the seeds.

Next, water the soil to keep it moist but not soaked. Now, cover the pot with a sheet of plastic and wrap the edges of the plastic around the top of the container to cover the entire surface of the soil. 

You can tie an elastic band around the pot to secure the plastic in place. You may also use a plastic bottle as a humidity dome. Cut out the bottom of the bottle and use it to cover the container of the seeds.

Egg Carton and Plastic Wrap

Egg Carton and Plastic Wrap

One common way to start seeds indoors is in an egg carton. If it’s made of paper, you can transplant the entire carton once your seedlings grow since the paper will simply decompose in the soil. 

To make a humidity dome from an egg carton, put a little soil in each of the cells. Plant a seed each and then water the soil. Use a plastic wrap to cover the entire carton, or better yet, use a resealable plastic bag. Some cartons may also come with its own cover made of plastic.

One disadvantage is when egg cartons get wet they may disintegrate. To avoid this, you can place the egg carton on top of a tray so you can move it when you need to. Another option is to use a plastic egg carton. This way, you won’t need a separate plastic wrap for the cover and you won’t fear breaking the carton when it’s wet.

Food Container with Lid

If you have a used plastic food container with a lid, you can also make a humidity dome with it. Just fill the bottom part of the container with soil. Make sure to leave space at the top for the air, water, and eventual seedling growth. Plant your seeds and water the soil. Now cover the container with the lid which will trap the moisture. A clear lid will provide the much needed light for the seeds once they start to sprout. 

Paper Towel and Plastic Bag

There is another way to make a humidity dome to germinate seeds without any container or soil. 

All you need is a paper towel and a plastic bag to store it. Just wet the paper towel until it’s damp but not soaked. It will serve as the growing medium in the place of soil. 

Then, put the seeds on the damp paper towel, wrap it around, then put them together inside the plastic bag. Seal the plastic bag to trap the moisture from the paper towel. When the seeds sprout, you can plant them directly into the soil.

Summary

While not always necessary, a humidity dome provides great benefits for starting seeds. It helps improve seed germination so you can get better results in a shorter time, and it doesn’t cost a lot even if you decide to buy one from a store. 

You can also make your own humidity domes at home easily. You need only to know the exact requirements of your seeds and monitor them to see when you must vent the humidity dome or remove it altogether. 

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