How to fill the planter?

July 15 2022 , 12 min reading time

#VALUE!

Found a new planter and wondering how to fill it properly? The way you fill a planter has a direct impact on how long your plants stay healthy and grow well. A poor setup — too little drainage, the wrong potting soil or plants placed too deep — quickly leads to root rot, drying out or poor growth. In this article, we explain step by step how to fill a planter correctly, which materials you need and what to look out for with different types of planters, from a standard window box to a large corten steel planter for outdoors.

What do you need to fill a planter?

Before you start, make sure you have the following materials to hand:

• Hydro granules or coarse gravel (for the drainage layer)
• Suitable potting mix or planter soil
• Lava rock as an alternative to hydro granules if needed
• Plants of your choice
• Water and a bucket
• Optional: slow-release granular fertiliser

Benefits of a correctly filled planter

• Longer-lasting plant growth: A good setup with drainage and nutrient-rich soil keeps plants looking their best for months longer.
• Less frequent watering: Hydro granules retain moisture during dry spells and release it gradually, so you need to water less often.
• Fewer diseases: Good drainage prevents water build-up and root rot — the most common cause of plant problems in containers.
• Stable plant growth: The right potting soil at the right depth creates a stable, nutrient-rich environment for the roots.

Step by step: filling a planter

1. Check the drainage: Check whether the planter has drainage holes. Without holes, water builds up and roots rot away. If the planter has no holes, drill several holes of approximately 20 mm in diameter in the base yourself.
2. Add a drainage layer: Fill the base with 2 to 3 cm of hydro granules or coarse gravel. This creates a water reservoir during dry periods and ensures good drainage when there is too much water.
3. Add potting soil: Fill the planter with the right potting mix to about 3 cm below the rim. Press the soil down lightly but not too firmly — the roots need to be able to grow through it.
4. Pre-soak the plants: Place the plants, still in their nursery pot, in a bucket of water. Leave them to soak until the root ball is thoroughly moist.
5. Remove the plants from the nursery pot: Carefully remove the plastic pot and keep the root ball intact. Damage the roots as little as possible.
6. Place the plants in the planter: Set the plant at the right depth — the top of the root ball should be level with the surface of the potting soil. This prevents the root ball and roots from drying out.
7. Top up with potting soil: Fill the space around the root ball with additional potting soil and press lightly so the plant stands firmly.
8. Water thoroughly: Water the planter generously straight after planting so the roots can establish immediately and the soil settles around the plant.

Ensuring good drainage

Good drainage is the most important foundation for a healthy outdoor planter. Most plants do not like wet feet — water that sits too long causes root rot and mould.

• Drainage holes: Always check whether the planter has drainage holes. If not, drill holes of approximately 20 mm in the base yourself.
• Hydro granules: Place a 2 to 3 cm layer of hydro granules on the base for a water reservoir and better drainage.
• Lava rock as an alternative: Lava rock also works well as drainage material, but replace it every four weeks to prevent calcium build-up — calcium is harmful to most plants.
• Saucer: Use a saucer under the planter to prevent splashing, but make sure it is not continuously filled with water.

Note for corten steel planters: Do not use hydro granules in corten steel planters outdoors. The combination of water and metal can accelerate the rusting process inside the planter in unwanted areas. Instead, ensure there are sufficient drainage holes in the base.

Hydro granules: how and when to use them?

Hydro granules are small balls of fired clay that absorb moisture when there is too much water and gradually release it again during dry periods. They create a natural moisture balance in the planter and are ideal for outdoor containers that experience alternating rain and dry spells.

• Use hydro granules as a drainage layer on the base of the planter — do not mix them into the potting soil
• A layer of 2 to 3 cm is sufficient for most outdoor planters
• For decoration, you can also place a layer of hydro granules around the plant on the soil surface — this also helps keep weeds at bay
• Do not use in corten steel planters

Choosing the right potting soil

The choice of potting soil has a direct impact on how well plants grow in a planter. Always use light potting mix or planter soil — never heavy garden soil, as this compacts quickly and restricts water drainage.

• Universal potting mix: Suitable for most flowers and garden plants in outdoor window boxes and planters.
• Planter soil: Richer in nutrients and specially formulated for use in containers. Ideal for perennials and patio plants.
• Cactus and succulent soil: For plants that require little water and need particularly good drainage.

Fill the planter to 3 cm below the rim — not right to the top. If the soil is too high, water runs straight over the edge and never reaches the roots.

Comparison table: filling materials for planters

Which plants to choose for a large planter?

The choice of plants depends on your personal taste, the location of the planter and the desired effect. Some guidelines for a great result:

• Choose a colour palette: Work with two or three complementary colours for a calm, cohesive look.
• Combine heights: Use a taller central plant as an anchor and fill the edges with trailing or lower-growing plants.
• Foliage plant as a base: A green foliage plant in the centre with flowering plants around it provides structure and colour throughout the year.
• Allow for growth: Plants keep growing — do not put too many plants in one container. Give each plant enough space to develop.
• Perennials for winter: Choose perennials or hardy varieties if you want the outdoor planter to stay green through winter too.

Comparison table: filling a planter for different container types

Frequently asked questions about filling a planter

How do I fill a large planter without it becoming too heavy?

Use a light potting mix or planter soil and fill the lower half of a large container with empty plastic bottles or polystyrene to reduce the weight. Place a drainage layer on top of this, followed by the potting soil.

Do I need to drill drainage holes in an outdoor corten steel planter?

Yes, drainage holes are essential. Without drainage, water builds up in the planter and leads to root rot. Drill several holes of approximately 20 mm in the base to ensure good water drainage.

How deep should I plant in a planter?

The top of the root ball should be level with the surface of the potting soil. Planting too deep leads to rot; planting too high causes the roots to dry out.

How often should I water plants in an outdoor planter?

This depends on the plant, the size of the container and the weather. Check the soil daily during warm periods — when the top centimetre feels dry, it is time to water. Hydro granules help keep the moisture level stable for longer.

Which potting soil should I use in outdoor window boxes?

Use planter soil or universal potting mix — never garden soil. Garden soil is too heavy for planters, compacts quickly and restricts water drainage. Planter soil is richer in nutrients and better suited for long-term use in outdoor containers.

Common mistakes when filling a planter

Common mistakes

• Not adding a drainage layer — leads directly to water build-up and root rot
• Filling the potting soil right to the top — water then runs straight over the edge
• Using garden soil instead of potting mix — too heavy and compacts quickly
• Using hydro granules in a corten steel planter
• Putting too many plants in one container — plants need space to grow
• Placing the root ball too deep or too high relative to the soil surface

Tips for long-lasting results

• Add slow-release granular fertiliser when planting for steady nutrition over several months
• Place a layer of mulch or hydro granules on the soil surface around the plant for decoration and moisture retention
• Refresh the top layer of potting soil annually to replenish depleted soil
• Place heavy outdoor planters on a firm, level surface for stability

Why Doika?

Doika specialises in high-quality outdoor planters and has extensive experience with the demands that outdoor containers must meet throughout the year. Whether you choose a corten steel planter or another style — we are happy to help you make the right choice. For questions about materials, sizes or maintenance, our customer service team is always on hand.

Conclusion

Filling a planter correctly starts with a good drainage layer, the right potting soil and planting at the correct depth. With the steps in this article, you create a solid foundation for your plants to thrive. Browse the full range of planters at Doika and find the perfect container for your garden.

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