Spring is in full swing, but you may notice that some of your carefully-sown seeds aren’t emerging as you hoped. A lack of germination could be due to planting seeds too deep or shallow in the soil. Finding the perfect balance is crucial for seedling success.
Direct sowing means seeding straight into your garden soil. Compared to starting seeds indoors and transplanting, this method has a few more risks because the seeds are exposed to an unpredictable outside environment.
Let’s dig into exactly how to determine the depth you should plant your seeds when direct sowing into your garden.
How Deep Should Seeds Be Planted?
When planting seeds, you should ensure that they are at a depth that is about twice their size. In other words, if a pumpkin seed is a half-inch long, it should be sown 1 inch deep. Tiny seeds like basil and lettuce should only have enough soil sprinkled on top of them to protect them from drying out or blowing away. It also helps to lightly press the seed into the soil surface.
Direct Sow Seed Starting Depth & Planting Steps
Plant seeds at a depth of twice their size, and protect tiny seeds from drying or blowing away.
Seeds sown in outdoor garden beds can be particularly difficult because you can’t control the outdoor seeding environment very much. You don’t have the cozy predictable environment of a windowsill or greenhouse, nor do you have the enclosed container of a seed tray.
Temperature, rainfall, storms, or pests are at the whim of Mother Nature. However, there are several factors you can control to make sure your seeds germinate quickly and evenly in outdoor beds:
|Seeding Factors You CANNOT Control||Seeding Factors You CAN Control|
|Pests that eat seeds||Seed depth and protection|
|Temperature||Use of row cover or plastic low tunnels|
|Rainfall||Drainage and moisture retention|
|Wind||Lightly pressing seeds into soil surface|
|Storms||Protecting seeds with mulch|
Let’s dig into the proper steps for understanding seed depth and direct sowing seeds into your garden this season.
Step 1: Estimate Planting Depth by Seed Size
Seeds must be planted at a depth that is proportional to their size, following the Golden Rule of Seeding.
Not all seeds are created equal! It is a common misconception that you can plant any seed at any depth and it will germinate. That method could cause a lot of headaches and wasted seed packets!
Due to the reasons below, the planting depth of a seed is always determined by its size. Whether you are planting in raised beds or containers, indoors or outdoors, don’t forget this rule:
Golden Rule of Seeding: Sow seeds at a depth that is twice their largest dimension.
Seeding Depth Cheat Sheet
Here are the estimated planting depths for some of the most popular direct-sown crops. You don’t need to pull out a tiny tape measure and magnifying glass to get the seed depth right. Eyeballing is generally fine as long as you follow the Golden Rule of Seeding.
|Crop||Estimated Planting Depth|
Step 2: Understand Seed Planting Depth Logic
The depth at which seeds should be planted depends on their size and the amount of food reserves they have stored.
It’s helpful to imagine a seed as a baby with its food in a package. This metaphor explains why a seed only has enough energy to be planted at a specific depth:
The embryo of the seed is a baby plant with all the genetic information needed to grow into a full-size crop. This tiny plant is curled up like a fetus in a stomach. It has baby leaves (cotyledons), a baby root (radicle), and an undeveloped stem.
The food inside the seed is called the endosperm. This starchy reserve of carbohydrates is what fuels the embryo on its journey up through the soil. A seed doesn’t have any leaves to photosynthesize and produce its own food as a grown plant does. That’s why the mother plant puts food reserves in the seed. If you plant a seed too deep, there won’t be enough food to nourish the baby plant before it reaches the soil surface and begins to soak up the sun.
The seed coat encases the endosperm and embryo. Proper moisture and warmth (as well as physical abrasion or microorganisms) trigger the seed coat to dissolve so the seed can germinate. If seeds are planted too shallowly, the seed coat may dry out, or it won’t have enough soil contact to wear it down and allow the embryo to break through.
Generally, smaller seeds like parsley and basil have little to no endosperm, which means they need to be planted shallowly.
Larger seeds like corn, pumpkins, and squash have a lot of endosperm, which is why they can be planted much deeper into the earth.
Step 3: Make the Right Size Dibble
Create dibbles using a long-handled garden tool or pencil to make indentations in the soil.
When you’re planting in the garden, bring along a long-handled garden tool or a pencil to create “dibbles.” Dibble is just a nickname for the indentation you make in the soil before you sow a seed.
If you get the dibble right, your seeds will fall perfectly into place, and it will be easier to cover them with just the right amount of soil.
There are two ways to make dibbles:
- Furrows: If you are planting seeds close together, such as lettuce mix, spinach, or peas, you will want to create a furrow. This is an elongated line or miniature ditch in the garden bed. For smaller seeds, drag a pencil or your finger lightly through the soil to create a furrow about ½” deep. For larger seeds, you can pull the back of a shovel handle through the soil to hollow out a 1″-2” deep furrow.
- Planting holes: For wider-spaced crops, you can create individual planting holes just like you would in seed trays. Press a pencil or your finger into the soil at the desired depth.
After making properly spaced dibbles, you can simply drop each seed into its pre-dibbled hole.
Step 4: Lightly Press Seeds Into the Soil
To prevent tiny seeds from blowing or floating away, gently tamp them down into the soil.
The wind is the greatest threat to tiny seeds like lettuce, basil, and many types of wildflowers. Because they don’t have much endosperm (food reserves), these seeds have to be sown near the surface.
You don’t want to cover them with too much soil because they may need some light to germinate. However, if the seeds aren’t pressed into the soil, they may blow away or float away with irrigation water.
The solution? Gently tamp them down.
Light pressure is key. You don’t want to compact the soil or push the seed any deeper. The goal here is to get a little seed-to-soil contact.
Step 5: Cover With Finely Sieved Soil or Compost
To cover the seeds after sowing, either flatten the soil surface or sprinkle compost over them.
The final step to sowing seeds outside is to cover them. If you use the dibble method above, you can simply brush your hand over the soil surface to flatten it. This will naturally push a little bit of soil into the top of the hole and keep the seeds perfectly tucked in at the right depth.
You can also use a container to sprinkle compost over the top of the seeds. Be sure that your compost or topsoil is strained so there are no large clumps. The finer the texture, the better!
Step 6: Maintain Continuous Moisture
Check outdoor seeds regularly to maintain consistent moisture.
Check outdoor seeds every day or so to ensure that the soil doesn’t dry out. This is especially important for carrots and other root crops. Your watering frequency can decrease after germination, but you want to maintain the most even moisture possible.
Big fluctuations in soil moisture can cause inconsistent growth and stunting as outlined in the video below.
To measure if you have enough moisture with your seedlings, stick your finger about 2 inches into the soil next to where you sowed your seeds. Follow the outline below depending on your findings:
- If your skin comes out clean or with a dusty residue, the soil is likely too dry. Water your seeds immediately!
- If your finger has a moderate amount of crumbly soil stuck to it, the soil moisture is probably good.
- If your finger comes out very dirty and muddy, you may be overwatering, or there has been a lot of rainfall. Let the soil dry out a little bit (but not completely).
Remember that water can displace seeds, so it’s very important that you irrigate lightly. Don’t blast newly sown seeds with a heavy burst of water, or they may float out of their planting holes.
Step 7: Thin Seedlings While Small
Thinning seedlings is crucial to avoid overcrowding and ensure optimal growth.
Once you’ve mastered seed depth, thinning is the next most important (yet often overlooked) part of direct sowing. If you don’t thin your seeds, your plants could end up overcrowded like people crammed into a subway. None of them will have enough room to breathe or grow to their fullest potential.
The best time to thin is when seedlings have developed their first set of true leaves. Check your plant spacing on the seed packet or website, then begin thinning:
- Choose the strongest seedlings and remove the smaller ones between them.
- Use needle-nose pruners or scissors to cut unwanted seedlings at the base.
- Avoid yanking or pulling them out because this can disrupt the root zone of the desired seedlings.
Before you sow seeds outdoors, take the time to properly prepare a fine seedbed that promotes ideal germination:
- Weed the bed and remove any plant debris.
- Add a thin layer of sieved (fine, chunk-free) compost.
- You can also lightly till the upper inches of soil.
- Rake the surface as flat as possible.
- Use a tape measure to lay out your plant and row spacing.
- Make a furrow or hole about twice as deep as the seed size, no deeper.
- Sow 15-20% more seeds than needed to account for any losses.
- Thin after seedlings develop their first true leaves.
When in doubt, never forget the Golden Rule of Seeding: Sow seeds at a depth that is twice their size!