Planting window boxes: when, how & tips


As a horticulture student I mainly studied crops and cultivation techniques. It fascinates me how many diverse plants can grow from small, nearly identical seeds.

Favourite fruit: blueberries, grapes, raspberries, pears
Favourite vegetables: mushrooms, peppers, kohlrabi, onions, garlic

Planting window boxes is a great way to grow plants while optimising space. Ornamental plants as well as herbs, fruits and vegetables all thrive in balcony planters.

planted flowers in window box
Window boxes are often planted with flowers, for a gorgeous display [Photo: Anna Nahabed/]

Whether vegetables, herbs, fruits or flowers, what you plant in your window box is up to you. But window planters have some advantages over regular pots on the balcony. They save space, have a better, back-friendly working height and allow the plants to get much more light. Read on to learn all about planting window boxes, including which soil to use and how many plants to plant.

When to plant window boxes

The best time to start planting your window boxes depends on the types of plants you choose. Frost-sensitive plants are best grown after the last frosts. These are typically over by mid-May, after which time you should not have to worry about frost at night. If you want to plant your window boxes sooner, cover the plants with fleece overnight to protect them from frost. There are also plenty of hardy balcony plants which you can plant in balcony planter pots before the last frosts.

Snow covers window box flowers
Do not grow frost-sensitive plants in your window boxes too early [Photo: Julija Erofeeva/]

The right soil for balcony planters

After choosing which plants you want to plant on your balcony, you need to get the right soil. With so many options to choose from, you will need to learn about the different plants’ nutrient requirements.

Soil for undemanding balcony plants

If you want to plant herbs like lavender (Lavandula angustifolia), thyme (Thymus vulgaris), rosemary (Rosmarinus officinalis) or sage (Salvia officinalis), we recommend a nutrient-poor soil with a loose, airy structure. Our Plantura Organic Herb & Seedling Compost is ideal for planting herbs and is completely peat-free, saving over 60% CO₂ during production compared to conventional compost. It is also suitable for balcony plants with low nutrient requirements, such as stonecrop (Sedum), flaming Katy (Kalanchoe blossfeldiana) and houseleek (Sempervivum).

For more nutrient-hungry herbs like basil (Ocimum basilicum), chives (Allium schoenoprasum), mint (Mentha) and parsley (Petroselinum crispum ssp. crispum), fertilise the soil a little. Alternatively, mix in a more nutritious soil such as our pre-fertilised Plantura Organic All Purpose Compost.

Balcony planter holds various herbs
Window boxes are great for growing not only flowers, but also your own fresh herbs [Photo: kay fochtmann/]

Soil for fruits and veggies in balcony planters

If you want to grow strawberries (Fragaria) or chillies (Capsicum), use a more fertile soil such as our Plantura Organic Tomato & Vegetable Compost. This contains organic fertiliser, so provides even the heaviest feeders with enough nutrients throughout the growing season.

Soil for summer flowers in balcony planters

For those wanting to plant flowers in window boxes, keep in mind that flowers have different soil requirements than other plants, like herbs for example. For various flower types, including petunias (Petunia), begonias (Begonia) or pot marigolds (Calendula officinalis), we recommend a high-quality soil, like our Plantura Organic Flower Compost. This soil is suitable for many flowering plants. As it comes pre-fertilised, it supplies your plants with nutrients right from the start and helps to promote long-lasting blooms.

Organic Flower Compost, 40L
Organic Flower Compost, 40L
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  • Perfect for all flowering plants in garden beds & pots
  • For beautiful blossoms & healthy plant growth
  • Peat-free & organic soil: CO2-saving composition

Tip: pay careful attention to your window box drainage. To ensure sufficient drainage, mix about a third of coarse material like expanded clay or perlite into the soil. Alternatively, create a drainage layer at least 2cm thick at the bottom of the planter.

How many plants per window box?

A 100cm large window flower box can hold roughly five to ten plants. Of course, this number can vary greatly depending on which plants you use. If, for instance, you want to grow a classic like geraniums (Pelargonium) in your window box, around five plants are ideal. If you plan on planting primroses (Primula), on the other hand, you can plant up to ten. If in doubt, research the plants you want to plant prior to planting your window box.

Tip: plants need plenty of root space, so sometimes less is more.

Person planting window box
The size of the balcony planter often determines what you can plant in your window box [Photo: Caterina Trimarchi/]

Planting window boxes: step-by-step instructions

First, set out all the necessary materials like soil, drainage material, window planter, a watering can and your plants. Gardening gloves are also handy. Then proceed as follows:

  • Check that the pots have enough drainage holes. Some plastic flower pot holes have to be drilled or punched out.
  • Place shards of clay at the bottom of the pot. This helps to prevent the drainage holes from becoming blocked and the soil from falling through.
  • Create a 3 – 5cm high drainage layer of expanded clay or gravel in the bottom of the window box.
Expanded clay at the bottom of balcony planter
Drainage is important as it helps to prevent waterlogging which can lead to root rot [Photo: Rudenko Alla/]
  • Optional: cover the drainage with fleece. This keeps everything separate and makes it easier for you to reuse the drainage material in the future.
  • Fill the window box about two thirds of the way with soil, leaving room for plants, of course. If you are filling a standard 80cm long window box, you will need just under 20L of our Plantura Organic Flower Compost.
Person shovelling soil into window box
Before you start planting window boxes, you will need to get the right soil for your plants [Photo: Rudenko Alla/]
  • Briefly submerge the root balls of your plants in water until they are completely saturated. This ensures that the plants have enough water at the start.
  • Cover the surface of the root ball lightly with soil. Press the soil down slightly to give the roots stability and fill any gaps with substrate. Remember not to plant the plants too deep.
Person planting flowers into balcony planter
Always water flowers generously after planting them in window boxes [Photo: Rudenko Alla/]
  • Leave about 3cm of space from the top of the window box so that the water does not spill over when watering.
  • Make sure to water generously after planting, especially if it is sunny. This will also flush out any last gaps in the soil.

Tip: if there is too much soil in the window box, push the soil down a little at the edge from time to time. This creates a watering rim which ensures that the water does not overflow later.

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