How Deep Should You Sow Seeds Indoors?
Are you planting seeds indoors this season, but aren't sure how deep you should be planting your seeds? The answer will depend on a few different variables. In this article, former organic farmer and gardening expert Logan Hailey shares the proper seed depth when starting your seeds indoors this season.
Planting seeds at the proper depth is essential for even germination and strong crops. When you are starting plants indoors, the amount of seed-starter mix covering each seed could make or break your seeding success:
- Seeds that are sown too deep may not have enough energy to germinate and reach the light.
- Seeds that are sown too shallow could dry out, blow away, or germinate with their fragile roots exposed above the soil.
Both situations can set back your spring gardening efforts. Striking a happy medium with seed depth will drastically improve your indoor seeding success. If you’re unsure about how deep to tuck in your seeds, here is everything you need to know about seed depth in indoor trays.
The Short Answer
You should plant garden seeds at a depth that is twice their largest dimension. In other words, if a pea or bean seed is ½ inch long, it should be planted an inch deep. If a lettuce seed is barely 2 mm across, it should be sown no deeper than a ¼ inch, with only a small sprinkle of soil on top. If you plant a seed too deep or too shallow, it may not have the proper conditions to germinate.
The Long Answer
Improper seeding depth is one of the most common seed-starting mistakes that beginner gardeners make. You should never sow a seed deeper than twice its diameter. In other words, seeds need to be planted about twice as deep as they are wide.
However, there is no need to take out a microscope and measuring tape. Simply remember these general rules:
Larger seeds like squash and beans tend to require deeper containers and more soil to ensure germination. Otherwise, they will dry out, get eaten, or germinate with exposed roots. Plant 2-3 times deeper than the width of the seed. These seeds are a little more forgiving if you plant them too deep, but they don’t do well when planted shallow.
Medium-sized seeds like brassicas and tomatoes should be placed in a hole that is twice their size. Gently backfill to ensure that the seed can easily reach the surface.
Very tiny seeds like basil and lettuce can be pressed into the soil surface and very lightly sprinkled with seed starter mix or “topper” mix (described below). These crops are the most sensitive to light and moisture. Sowing too deep will kill the seed.
Risks of Sowing Too Deep
Seeds only have a certain amount of food storage to sustain them through germination. Planting too deep can result in the baby seedling dying before it reaches the surface.
Small seeds like celery, lettuce, basil, and thyme are particularly sensitive to deep sowing. Thankfully, larger seeds like peas and cucumbers are more forgiving.
If you sow a seed too deeply, you risk:
Seeds have to live off of their food reserves (endosperm) until they can reach the surface and start photosynthesizing in the sunlight. When you plant too deep, the seed may sprout but never germinate because it ran out of food on the way up. This means you won’t see any baby green seedlings at all.
Unless you are using a germination heating mat, the lower layers of seed-starting pots can also be colder, leading to reduced germination rates of warm-weather crops. Underwatering, overwatering, improper soil temperatures, expired seeds, and seedling diseases like damping off can also cause poor germination.
If a baby had to crawl through several feet of passages before birth, they would probably be born exhausted. It would take a lot of time for the child’s body to recover, and shift focus back to growing bigger.
The same is true of a seed. Seeds that have to work super hard to move through several inches of soil are usually weak and stunted by the time they burst up through the soil surface.
If your seed-starter mix or seeding containers lack drainage, deeply sown seeds could disintegrate or fall victim to disease before they can germinate.
Overwatering and poorly drained seed starter mix are usually the culprits. Out in the garden, seeds can also rot under excessively moist conditions or compacted, heavy clay soil.
However, you also need to take care not to sow seeds too shallowly. When starting seeds indoors, the upper layer of soil mix dries out much more quickly than the rest of the container.
Risks of Sowing Too Shallow
Planting a seed too shallow can cause low germination or seedling death. Seeds that aren’t tucked deep enough into the soil often dry out before they can sprout. They are also vulnerable to injury or displacement from pests, diseases, flooding, and wind.
If you sow a seed too shallow, it is more susceptible to:
The movement of air or wind over the seed-starting tray means that the soil mix dries out more quickly on the surface. Seeds are very sensitive to moisture during their early stages. When a seed is exposed to dry air during the germination period, it could stop germination, slow the growth, or kill the plant altogether.
When a seed doesn’t have proper seed-to-soil contact, it is more prone to floating away with irrigation water. When you water your seed trays, the seed could become dislodged and drift out of the container. Even worse, a fresh young root system can easily be disrupted by water when it isn’t fully anchored in the soil.
Depending where you are starting your seeds, air drafts and fans could blow away very lightweight seeds that are sown shallowly on the surface. While this is typically a problem with outdoor garden winds, it’s still something to be aware of in your home or greenhouse.
A seed that isn’t properly “tucked in” becomes an easy target for hungry insects. In a nursery or greenhouse, rodents also pose a significant threat to seeds that are sitting too close to the soil surface. For example, cucumber seeds are absolute magnets for hungry mice and voles. Sometimes, they’ll even dig the seed out of the soil!
Shallow seeding is most problematic for large-seeded crops. Imagine a pumpkin seed trying to germinate on the soil surface. It might dry out or its fragile developing roots would be exposed to the scorching sun. These larger seeds need the moist darkness deep in the soil to properly sprout.
On the opposite end of the spectrum, some tiny seeds need filtered light to trigger them to break dormancy and germinate.
They can actually benefit from shallow sowing as long as they are pressed into the soil and have a little bit of seed-starter dusted over the top. For example, lettuce, poppies, snapdragons, and many wildflower species benefit from shallow seeding.
What About Direct Sowing?
Some seeds will grow on the soil surface, but most garden seeds require a layer of soil over them to protect the seed from drying out or getting eaten by pests.
Of course, there are robust seeds that will sprout just about anywhere. Seeds that tolerate shallow sowing are often those that are closely related to their wild plant ancestors.
If you scatter grass seed or wildflower seed on a bare patch of soil, the majority of them will germinate if there is enough rainfall. However, vegetable and fruit seeds tend to be much more finicky. You will have the most success by seeding them at the proper depth in seed-starter mix or rich garden soil.
Creating a Seed “Topper” Mix
A fluffy seed “topper” mix is one of the easiest ways to buffer your garden against seed depth mistakes. This is a very lightweight, finely-sieved blend that can be dusted on top of your seed-starting trays after sowing.
Instead of covering seeds with the same seed-starter mix that they are planted in, a “topper” mix has the added benefits of:
- Extra fine, sieved materials
- Lighter weight on top of the seeds
- Moderates moisture levels to prevent drying out
- Rapid water filtration
- Allows filtered light in (if needed for germination)
Professional Vegetable Farmer Recipe
- 1 part grade 1 (fine) vermiculite
- 1 part fine grade perlite
- 1 part finely sieved compost
Mix together and lightly moisten. Fill your seed-starting containers with the usual seed-starter mix, then sow your seeds at the proper depth. Use a small cup to sprinkle the “topper” blend over the seeds. This is most useful for small and medium-sized seeds ranging from basil to lettuce to peppers to broccoli.
For a simpler option, you can also run your seed-starter mix through a sieve made of fine hardware cloth. As you break up the larger chunks of soil, you ensure a more even coverage of your seeds.
Importance of Sowing at Proper Depth
Seeds need to be planted at a specific depth because they have evolved to germinate under specific temperature, light, and moisture conditions. A squash seed must be planted about 1 to 2 inches deep in the soil because it needs plenty of warmth and moisture.
But a basil seed must be carefully covered with a thin layer of soil because the tiny seeds need light to germinate and they aren’t large enough to emerge from deep in the ground.
Every crop has different genetics and growth requirements, but they typically share the same basic seed structure. When you understand the biology of a seed, it’s so much easier to determine the proper depth.
Let’s get nerdy for a second so we can see what is really happening when we tuck a seed into seed-starter mix to prepare for the gardening season.
Seed Biology 101
Imagine a seed as a baby in a package with some food. This is a simplified analogy based on human perceptions, but it is still scientifically-accurate for the majority of our garden crops. The three parts of a seed are:
- The “baby” = the plant embryo, or the young plant itself
- The “package” = the seed coat, which protects the embryo from harsh environments
- The “food” = the endosperm includes starch and protein-rich food reserves to nourish the embryo throughout the germination process
Seed Depth in Nature
Before the seed was born, a mother plant put all her energy into growing flowers. Those flowers were pollinated with the pollen from another flower, creating unique genetics. Then, the pollinated flower grew into a fruit.
Inside the fruit there could be a single seed or a thousand seeds, depending on the plant. In order to ensure the survival of its babies, the plant ensures that every seed has its own protections to survive out in the big world.
In the wild, a tomato fruit would ripen and get eaten or fall to the ground. The seeds inside that tomato would eventually find their way to the soil through wind or a pile of animal poop.
The seeds that are lucky enough to end up buried in ideal conditions with moisture and warmth would germinate into new tomato plants to restart the cycle of life. However, the seeds that end up buried beneath piles of soil or sitting on the surface of dry rock probably won’t ever germinate.
Seed Depth in Domestication
Through plant breeding, greenhouses, and garden innovations, humans have dramatically increased the chances of success that each seed has. In fact, many of our vegetable seeds can have a 90-98% germination rate in seed-starter trays. The key is giving each seed the best conditions possible.
How do you know the best depth for a seed? Look at the size of the endosperm (the “baby’s” food storage)!
|Seed Type||Seed Description|
|Pumpkin or Squash||A pumpkin or squash seed has a large amount of endosperm. This is why these seeds are so protein-rich and nutritious to eat. Because the seed has so much energy stored up, it can be planted deeper.|
|Legumes and Corn||A bean or corn seed has a fair amount of endosperm. The starchy yellow part is easily recognizable. These seeds are sown about ½ inch to ¾ inches deep.|
|Tomato and Pepper||Tomato and pepper seeds have very small endosperms that are considered “non-starchy”. This is why the seeds appear flattened. They need to be planted fairly close to the soil surface, or they will run out of energy during germination.|
|Non-endosperms||Some plants, like orchids, have no endosperm at all. They cannot germinate unless they are right on top of the soil surface and connected with a special type of mycorrhizal fungi to help them sprout. However, even the smallest vegetable seeds like basil and lettuce have food storage that allows them to germinate beneath a small amount of soil.|
Ultimately, the protection and food stores in a seed are what make it possible for us to save, store, and transport them. Seeds are truly fascinating because they have evolved to stay dormant for long periods of time until the proper conditions arise.
Maximize your seeding success by planting your vegetable seeds twice as deep as they are wide! If you want to get a head-start on your season and have the best germination possible, learn more in our complete guide on How to Start Vegetable Seeds Indoors.