Planter boxes are a great choice for urban gardeners looking to grow flowers in a small space like a windowsill or balcony. However, if you plant to use these boxes to grow flowers, you need to consider how deep you need them to be.
A planter box for flowers needs to be approximately 8 inches (20.32 cm) deep. Since most types of flowering plants have shallow root systems, they don’t need very deep planter boxes in order to grow and thrive. However, if you’re growing perennials, use a 14-inch (35.56 cm) deep planter.
In this article, I’ll look at why planter boxes need to be at least 8 inches deep and how to choose the best planter depth depending on the types of flowers you wish to grow. I’ll also provide tips on how to ensure you can grow healthy plants when they’re in planter boxes.
Why Your Planter Box Needs to Be 8 Inches (20.32 cm) Deep
If you’re growing flowers in a planter box, you need to accommodate their root growth to ensure you give the flowers enough space to absorb nutrients, oxygen, and water from the soil.
Here are the main reasons why your planter box should be at least 8 inches (20.32 cm) in depth:
It Provides Adequate Room for the Flower’s Roots
If the flower’s roots are restricted, this will prevent them from growing properly. Without healthy roots, flowers won’t survive.
It Acts as a Temperature Buffer
A container that’s around 8 inches (20.32 cm) in depth ensures that there’s more soil between the bottom and sides of the container, protecting plant roots from temperature extremes.
Flowers Will Get the Moisture They Need
A shallow planter box will need and hold less water. This ensures that you don’t give your flowering plants too much water, an issue that can lead to root rot.
How to Choose Your Flower’s Planter Box Depth
You have to adjust the depth of your planter box according to the type of flowers you wish to grow. While it isn’t the only consideration, a key determiner of your planter box depth will be how long your flowers live.
Are they annuals, biennials, or perennials?
Annuals
Examples of popular annual flowers include:
- Marigolds
- Zinnias
- Sunflowers
Annual flowers have shallow root systems, so they’ll be able to grow in planter boxes that have approximately 8-inch (20.32 cm) depth. However, this also means that you’ll usually have to water them more than perennials and biennials to make up for their smaller roots.
Annuals are a great choice for planter boxes because they can grow easily in any type of container. You can plant them in window boxes, baskets, and hanging planters.
Annuals do well in smaller containers as their roots don’t need lots of space. You can also pack the plants tightly because of their shallow roots, which is great if you want to fill your planter box with lots of colors.
Some annual flowers can even grow in about 6 inches (15.24 cm) of soil. These include pansies and sweet alyssum.
Biennials
Biennial flowering plants include black-eyed Susans, hollyhocks, and foxglove. Biennials live for two years, which might make them more appealing than annuals, which only live for one.
When choosing enough depth for your planter box, aim for a depth of about 12-16 inches (30.48 – 40.64 cm), depending on the type of plant. For example, hollyhocks need a container that’s at least 16 inches (40.64 cm) deep, while foxglove requires a 12-inch (30.48 cm) container.
Perennials
Perennials, such as astrantia and daylilies, have larger bulbs, which means they need a deeper planting box – aim for a depth of 14 inches (35.56 cm). This will ensure that their roots can absorb water and nutrients from the soil to grow healthy.
Since perennials have larger root systems than annuals, you need to ensure that there’s also enough space between each plant.
That said, there are some perennial flowers that have shorter root systems and perform well in 6 to 8 inches (20.32 cm) of soil. Examples include nasturtium and dwarf marigolds.
If you’re interested in planting marigolds, you should read my guide on whether their seedlings should be pinched back: Should Marigold Seedlings Be Pinched Back?
Why You Should Avoid Planter Boxes That Are Too Shallow
While some plants, such as cacti, can grow successfully in planter boxes with a depth of as little as 3 to 4 inches (7.62-10.16 cm). Most other plants need deeper soil to grow correctly.
Unfortunately, many people undersize their planters.
Here are a few indicators of a shallow planter box:
Rootbound Plants
A major cause of rootbound plants is a small planter. While they might suffice in the seedling stage, they need to be swapped as the plants mature. Rootbound plants will usually develop symptoms similar to underwatered plants as the roots struggle to take in enough water.
Unstable or Shaky Plants
Smaller pots offer less opportunity for your plant’s roots to gain purchase in the soil. This makes them much less stable.
Cramped Roots
You’ll see that the plants will have stunted growth, and their leaves will become yellow because the plants can’t absorb enough nutrients from the soil.
Dry Soil
If you’re only supposed to water your plant every few days, but its soil dries out quicker than the schedule, this could be a sign that its roots are overdeveloped, and your plant should be in a larger container.
Why You Should Avoid Planter Boxes That Are Too Deep
While it’s easy to assume that giving your plants a deeper planter box is better than one that’s too shallow, a planter box that’s too deep can also cause problems, so it should be avoided.
Here’s why:
Plants Can Become Waterlogged
A large planter box’s soil can hold much more water than a smaller one. This could cause it to remain wet for longer, which puts the flowers at risk of diseases such as fungus, root rot, and mold growth.
Big Planters Can Become Too Heavy
A too-large planter box will require a lot of soil, which makes it heavy to move around. Wet soil will make it even heavier!
The Soil Can Become Loose
Similar to having a shallow planter box, a very deep one can cause the soil to lose its firmness around the plant’s roots.
Plants Will Compete to Reach Nutrients
If your flowers are in a planter box that’s too deep, they might begin to compete for nutrients. While the soil itself might hold enough nutrients, some of it could simply be too far away for the plants to make use of.
How to Grow Flowers in Planter Boxes
Once you’ve ensured you’ve got the right planter box depth, there are other important tips to follow to ensure the healthy development of your flowers so you can see them bloom.
Here are some essential ones to bear in mind when setting up your planter boxes:
Line With Weed Matting
Lining your planter boxes with weed matting encourages drainage and prevents weed growth. I’d recommend purchasing the Agfabric Landscape Fabric Weed Barrier from Amazon. It’s completely free of chemicals, so it won’t harm the soil or reduce the nutrient content that plants need.
Buy Potting Soil for Your Flowers
Fresh soil helps your flowers store and release water effectively. It also gives your plants a secure anchor. I’d recommend Espoma Organic Potting Soil Mix from Amazon. It contains ingredients such as perlite and sphagnum peat moss to encourage water drainage.
Add Organic Matter to Prevent Compaction
The soil in your planter box can become compacted, which reduces how effectively your plants can access nutrients and water from the soil. This is why adding organic matter, such as compost, to it every year is beneficial.
Fill to 1-Inch (2.54 cm) Below the Rim
Leaving an edge gives you enough space for watering your flowering plants, so make sure you choose a large enough pot that caters to the root growth of your plants and how much soil they require.
Encourage Effective Drainage
Making drainage holes in your planter, if it doesn’t have any, is essential and will encourage the effective drainage of moisture from the soil.
Plant Flowers 3-4 Inches (7.62-10.16 cm) From Each Other
Keeping flowers a few inches apart will ensure their roots have enough space to grow into the soil. If you’ve overcrowded your plants, a way around this is to prune them.
Put Plants With Similar Soil Depth Requirements Together
Make sure that if you’re adding perennials and annuals to the same planter box, you’ve checked how deep the planter needs to be for all of them so you can group plants that need similar depths together.
Final Thoughts
If you want to grow beautiful flowers, whether annuals or perennials, you’ll need to ensure your planter box has the correct depth. A depth of approximately 8 inches (20.32 cm) is ideal for most annuals, while perennials require a depth of approximately 14 inches (35.56 cm).
Ensuring the correct depth is important to encourage healthy flowers to grow.
It also prevents issues that can arise from using a shallow or very deep planter box, such as:
- Heavy soil
- Stunted growth
- Loose, unanchored plants
- Waterlogged soil
