Humidity Domes for Seed Starting

March 29, 2024

Want better results when starting seeds indoors?

When starting seeds indoors, it's important to manage several environmental of those being humidity during germination and sprouting.

When seeds germinate, and the initial but shoot and root initial (radical) emerges, that seedling doesn't yet have root branching or root hairs with which to take up water and nutrients yet. It's surviving off of the resources stored in the seed.

So as the seedling emerges it can quickly shrivel and die if the relative humidity in the space is too low (especially if temperatures around the seedlings are warm or hot due to close grow lights).

You'll need to keep relative humidity (RH) pretty high - around 60% - to keep your young seedlings healthy. You'll need to maintain this RH for several weeks, at least until the seedlings put on their first set of true leaves. Now, this is relative also the species you're growing. Even so, when you're germinating cactus seeds and seedlings, you'll still find higher humidity is going to get you the best results.

Same goes for when you're germinating microgreens seeds.

So, how do you manage to temporarily keep RH high around you seedlings to get them off to a good start? Use a humidity dome!

PC: Leslie F. Halleck

Many humidity domes have top or side vents. After the seeds have germinated and begin growing or cuttings begin to root, you can open the vents to slowly reduce humidity. If too much water begins condensing inside the dome, or you keep the humidity dome on too long, fungal diseases or rot could set in. So if you're using a humidity dome with no vents, you may need to take it off for short periods of time now and then.

Some humidity domes come in taller sizes with a vent you can open and close to conserve or vent moisture and heat. I prefer to use tall humidity domes with vents because it makes moisture management easier, allowing you to grow the seedlings under cover for longer.

If you're germinating a smaller group of seeds, say in a single pot or cup, you can use clear plastic cups turned upside down, or even plastic bags, as a humidity dome. You can also use glass jars or cloche.

If you want to learn more about successful seed starting, check out my book "Plant Parenting" where I guide you step-by-step!

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