How to Cover Seeds to Boost Germination – and Which Ones Need Light, Instead

Moisture and warmth are vital to achieving strong germination when sowing seeds. A crucial and easy way to get those two factors right is covering seeds for germination, but what you put on top will depend on the type of seed.

The options on offer include soil, vermiculite, and various types of plastic covers. What you should opt for varies depending on the size of the seeds, while in many scenarios, you will use a combination of them.

I regularly used all of the above when growing vegetables, flowers, and herbs from seed as a professional gardener. This useful guide to covering seeds for germination reveals why it is important, the needs of different seeds, and the best materials for the job. This knowledge will definitely help you germinate seeds more successfully.

An Expert Guide to Covering Seeds for Germination

Working in public and private gardens, I led large-scale propagation regimes that grew many hundreds of different plants each year from seed. A key to success was tailoring the right environment for each seed depending on its needs.

The variables included the temperatures for germination, whether the seeds were covered with soil or vermiculite, and whether a plastic cover was used to create a vital mini-greenhouse for them.

There is equipment that makes a difference between great and disappointing germination. An example of a complete setup to give seeds heat and light is this seed starting kit at Amazon, which has a heat mat, grow light, timer, and humidity dome.

How Important Is Covering Seeds for Germination

Covering seeds is paramount in creating perfect conditions for seeds to germinate. This is because, for the best results when sowing seeds indoors and outdoors, all seeds want warmth, moisture, and oxygen to germinate.

But what about light? Light is only a trigger for some seeds, and those that require light don’t want to be covered, as they will struggle when buried underneath a layer of soil.

Lotte Berendsen, a gardening enthusiast, writer, and plant expert at PlantIn, adds that covering seeds for germination is crucial for those where light isn’t an important triggering factor.

‘Many seeds require a dark, moist, and warm environment to germinate, and covering them can help you reach that environment more easily,’ she says. ‘Covering your seeds will help retain moisture and warmth, providing the perfect conditions for germination.’

How deep you sow seeds varies. Those that need light are sown on the surface and left uncovered, or given a fine covering. Ones that prefer darkness are planted deeper, from a half-inch to an inch deep, depending on their size.

To retain warmth and moisture, gardeners create a humid environment by covering seed trays and pots with plastic lids (also known as humidity domes) or plastic bags. This technique can help boost germination for all seeds.

Lotte Berendsen

Lotte is a gardening enthusiast, writer, and plant expert at PlantIn, where she provides customers with detailed care and treatment plans.

Which Seeds Don’t Need Covering?

Covering seeds for germination comes with many benefits, but it shouldn’t be done with all seeds you want to grow for flower beds or vegetable gardens.

Treating all seeds the same is a seed-sowing mistake that badly affects your success. What is important to gardeners is understanding the needs of different seed varieties.

‘Some seeds require more light and oxygen, which means that covers that create darkness are not the best option, adds Lotte. ‘Some examples include: lettuce, basil, oregano, strawberries, petunias, and lavender.’

On top of those outlined by Lotte, the likes of coleus, snapdragons, alyssum, scabious, gaillardia, and nicotiana also require light to germinate and will not sprout when covered with a thick layer of soil.

Most other seeds are best covered with soil to ensure good germination rates. The best way to ensure you are doing it right and prepare for seed sowing is to consult the seed packets. These will tell you the depth at which to plant the seeds, including whether they want covering or not.

What to Cover Seeds With

Seeds that require light to germinate are usually tiny. These are best pressed into the surface of the soil, and, if anything, given a fine layer of vermiculite or soil. This light dusting can be enough to allow light to penetrate and keep them moist, but it is not always necessary.

It can be beneficial to cover them lightly with soil or grit when sowing seeds outdoors. For indoor sowings, pushing the seeds into the soil surface and then covering the trays with plastic can be the best approach to boost germination.

A clear plastic lid or a plastic bag helps provide a warm environment and allows essential light get to the seeds. You can get a pack of clear humidity domes for seed trays at Amazon, or buy small nursery pots complete with humidity domes, also at Amazon, depending on the sowing you plan to do.

For seeds that don’t need light, plant them at the recommended depth and cover them with fine seed compost or vermiculite.

Seed compost, like this seed starting mix at Burpee, is always the best soil to start seeds in, as it has a fine texture. This texture is not only good for developing roots, but it also makes it easier for young seedlings to push through the layer when they germinate.

A layer of lightweight vermiculite can be put on the surface of the soil to keep the compost moist. You can get a bag of organic vermiculite at Burpee, which can be mixed with soil to make a lighter mix or added as a layer on the top of seed trays.

The trays or pots can also be topped with a plastic cover to retain warmth and boost the germination rates.

FAQs

When Do You Take the Cover off the Seedlings?

It is advisable to remove plastic covers, such as domes or bags, from the seedlings once they have emerged through the soil. Leaving the covering in place continues to create a moist environment, which can put seedlings at risk of damping off, which is a fungal disease that causes them to collapse and die.

Other important tasks can make a difference between seeds germinating and not, and whether these are necessary is often revealed on seed packets.

It includes whether seeds need cold stratification, where they are subjected to cold to mimic their natural environment and trigger growth. Gardeners can stratify seeds by sowing them outside in the fall to germinate in spring, or by putting seeds into a refrigerator for several weeks.

Scarifying seeds, or nicking them and making a small cut in their coating, as well as soaking seeds before planting, are also beneficial when dealing with seeds that have hard outer shells. They will germinate without this extra attention, but doing it can speed up the process.