7 centimeters is the only thing that matters for bulbs, seedlings, and borders in many gardens. Not expensive tools or rare plants. Some experienced gardeners say it’s just a fad, while others say it changed their plots. The conflict between habit and horticultural science is now just below the surface.
Why 7 cm is the depth that keeps gardeners on edge
Most people think that depth in the ground is a rough guess. You stick a trowel in and say, “That looks fine,” and then you wait and see what happens. But more and more practice and soil science point to a small area that keeps coming up: about 7 cm below the surface.
Seeds and young roots are in a sweet spot at about 7 cm deep: they are safe but still able to breathe.
Many gardeners still rely on their gut feelings. To be safe, they bury bulbs “nice and deep.” For quick sowings, they only scratch the surface. The result is that the seeds don’t all sprout at the same time, the stems are weak, and the beds never quite fill out. A lot of people don’t like the idea of 7 cm because it seems fussy and obsessive. But tests in both vegetable plots and ornamental borders keep coming back to this same small number.
A short distance that changes how people plant things
Get a mixed border in the fall. I’m planting tulip bulbs, young perennials, and salad seedlings that will stay alive over the winter all at once. Roots can start working right away when they are put in the ground with their base about 7 cm below the final soil level. This gives them a safe place with enough warmth and oxygen.
The soil gets heavier and colder when you drop them a few centimeters deeper, especially on clay. The growth slows down, and the rot becomes more likely. If you set them too shallow, they will dry out or freeze when the first hard frost comes. There isn’t much of a difference on paper. It’s almost impossible to see. It is not at all on the plants.
Why a lot of gardeners still don’t trust the tape measure
Tradition is a big part of why people don’t want to change. A lot of people learned how to plant from a parent or grandparent, not from a graph of soil temperature. The old saying was “twice the height of the bulb” or “a good spade’s depth.” That rough-and-ready advice helped people build gardens for generations, so a strict 7 cm can seem very strict.
But modern gardens have new problems to deal with, like soil that has been compacted by mowing too often, hotter summers, hosepipe bans, and smaller, more densely planted areas. In these conditions, accuracy is more important than it was in a loose, well-manured field from years ago.
The 7 cm rule isn’t really a trend; it’s more of a practical way to deal with smaller spaces, harsher weather, and stressed soils.
The soil science that is 7 cm below the surface
This depth isn’t just a random number that was chosen. It goes back to what really happens in the top layers of soil over time when temperature, moisture, and living things are measured.
What is really going on just below the surface
The first 3–4 cm of soil in many temperate gardens changes a lot with the weather. The sun bakes it, the frost holds it, and the wind takes the moisture out of it. The soil stays cooler and often denser when you go deeper than 10 cm, which slows down early growth.
About 7 cm is in the middle of those two extremes. Here, the soil stays aerated enough for roots to breathe, but it also protects them from sudden changes in temperature. This is the best place for earthworms and a lot of helpful microbes to work. They break down organic matter, let nutrients out, and make holes that roots can easily follow.
Microbes, warmth, and moisture: the quiet alliance that plants need to grow
Moisture stays longer after it rains at this depth. The top layer dries out, but the bottom layer stays wet without getting too wet. Microbial life keeps going even when the air gets cold because the temperature changes less quickly.
In a normal garden bed, the 7 cm band is usually where the most active mix of fine roots, fungi, bacteria, and soil fauna is found.
Plants that get to this layer early usually grow better on top later. They hold on better, deal with wind better, and bounce back faster from times of drought or heavy rain. For gardeners who want thick borders and dependable crops, that underground stability is worth more than any new type.
What happens when you plant too deep or too shallow
Ignoring depth doesn’t usually kill everything right away. It just makes a pattern of “almost good” results: rows that are uneven, bulbs that bloom once and then disappear, and beds that never fully knit together.
The usual problems with traditional planting
People often think that deep planting is safer, especially before a cold winter. But when roots are too low:
- The amount of oxygen in the air goes down, especially in heavy, clay-rich soils.
- Water stays around, which makes it more likely that rot will happen around fragile root collars.
- Soil stays cold longer in the spring, which slows down growth.
If it’s too shallow, a new set of problems arises:
- The first warm spell dries out the roots.
- Frost can pull seedlings and bulbs out of the ground.
- Plants sway in the wind, breaking off new root tips.
How 7 cm helps and encourages your plants
When you plant bulbs, seeds, or small plugs with their base about 7 cm down, they are below the worst of the surface stress but close to the richest organic layer. A lot of gardens have compost or mulch on top, and then a darker, crumbly area full of life below. That is where roots can get food and hold on.
Seven centimeters is a good middle ground: it’s deep enough to protect, but not so deep that it slows down plants.
The result is that seedlings grow faster, lines of seedlings are more even, and perennials grow bigger instead of staying small for a year. In mixed beds, the difference often shows up as planting that is smoother and denser, with fewer bare spots where weeds can grow.
How to use the 7 cm rule in your own garden
This method works best in late fall and early winter. The soil is still workable, and beds are being cleared. Many gardeners are putting in last-minute bulbs or cool-season crops.
A practical guide to where 7 cm makes the most sense
| Type of plant | How 7 cm applies |
|---|---|
| Root vegetables (carrots, parsnips, turnips) | Sow seeds so they end up roughly 7 cm under the surface in light soil; a little less in heavy clay. |
| Young vegetable plugs | Plant so the collar of the plant finishes around 7 cm below the final soil or mulch level. |
| Spring-flowering bulbs | Rest the base of the bulb about 7 cm down, adjusting slightly for very large or very small bulbs. |
| Perennial divisions | Position crowns just above the 7 cm zone, with the bulk of roots reaching into it. |
Divisions that last foreverPlace crowns just above the 7 cm mark, where most of the roots can reach them.
This might seem like a lot of work for gardeners who are used to just looking at things. Once you can picture what 7 cm looks like on your trowel, it becomes second nature.
Easy ways to stay on track
- Use permanent marker or tape to mark 7 cm on the side of your trowel.
- Put a short, weatherproof ruler or wooden stick in your trug so you can check things quickly.
- Measure once for each new kind of plant, and then use your eye for the rest of the row.
- If you add 3 cm of compost, plant 4 cm lower to keep the base at 7 cm.
- The goal is not to get every seed perfect, but to make sure that all of your beds are the same so that roots always reach that active soil layer.
What gardeners say they see after switching to 7 cm
In a lot of small gardens in Britain and North America, planting more carefully for one or two seasons has already changed people’s minds. People who tried 7 cm out of curiosity often say the same things happen.
From rows that aren’t straight to growth that is stronger and steadier
When root crops are planted at this depth, they tend to germinate more evenly. Rows grow straighter, thinning takes less time, and roots grow more evenly. With bulbs, stems hold up better in the wind, and flowers come back every year instead of fading after the first bloom.
There are usually fewer weeds in mixed ornamental beds. When gardeners get the depth and mulch just right, they make a more stable top layer that blocks out seedlings that are trying to take advantage of the situation. That means you won’t have to weed as much, and you’ll have more time to shape the border’s overall design.
Soil and crops will benefit in the long run.
Doing this every season has a small but strong effect. The 7 cm band is a busy place with roots and life. There, organic matter is always being added and broken down, which makes the soil structure better.
As the soil at this level gets richer and looser, each new generation of plants grows faster and needs less watering and feeding to survive.
This also fits perfectly with crop rotation. When all the plants settle into the same target layer, the life in the soil changes more quickly and recovers more quickly between crops. As long as organic matter is put back into the busy 7 cm zone on a regular basis, salad greens, root vegetables, flowering bulbs, and small shrubs can all live in the same beds without tiring out the soil.
More information for gardeners who want to know more
How the type of soil changes the 7 cm talk
The best place to start is with the 7 cm rule. If you have sandy soil that drains quickly, going a little deeper can help keep moisture in the soil for longer. When the clay is sticky, going a little shallower can help the roots get more air. The idea is still the same: look for the active, living band of soil that isn’t too dry or too wet.
After it rains, gardeners can do a simple test by digging a spadeful and seeing where the soil feels crumbly instead of puddled or bone dry. That texture is usually the best place for roots to work.
Picture two gardens that are 30 cm apart.
Imagine two people who live next door to each other on a typical suburban street. One person plants everything by habit, either pressing bulbs “nice and deep” or scattering seeds under a thin layer of soil. The other person takes ten minutes to mark 7 cm on a trowel and move their planting.
Their gardens might look the same from the sidewalk by the end of spring. As you get closer, you can see the differences: the rows of carrots are fuller, the tulip stems are stronger, and there are fewer gaps in the perennial border. The two gardeners bought plants that were very similar. The difference between their results is only 7 cm.
You don’t need new tools or complicated methods to make that small change in the soil. It just wants a measured gesture at planting time and a willingness to let evidence, not habit, decide where roots start their lives.
