The main ingredient to use to fill your planter box should be good quality raised garden mix. This mix will contain a mix of organic matter and soil or sand which will hold the structure for your plants. It will also contain slow release fertilizer. If you are planting vegetables then you can plant straight into this mix.
This article will explore all you need to know about how to fill a planter box at home. Find out how to prepare your soil and how to line your planter box.
Simple steps to fill a planter box
- Place weed matting on the bottom of the planter box if you have invasive weeds like couch grass.
- Layer bricks, large rocks or logs on the bottom of large planter boxes to fill the space cheaply and for drainage.
- Cover the rocks with a layer of river sand.
- Fill the planter box with good quality raised garden mix or all-purpose potting soil.
- Mix compost and aged cow manure through the top layer of the planter box.
- Plant vegetables, annual flowers or citrus in the planter box.
- Cover the soil with mulch and water it in.
Drainage for planter boxes
Most planter boxes will have an open bottom. Place a layer of weed matting on the bottom of the garden bed to prevent weeds from growing through. This will allow water to drain out easily.
Place your raised planter box straight onto garden soil or landscaping rock to allow the water to drain out of the bottom. There is no need to put gravel or stone inside your garden bed. Raised garden beds will drain well without this and it will give you more room for soil.
To find out more about the best depth for planter boxes for vegetables, check out my previous article.
How to line your planter box before filling
Planter boxes can be lined with garden fabric or weed matting. This will drain well but keep the soil from being in contact with the walls of your planter box and will stop weeds from growing through the bottom off the bed. Check out this one from Amazon.
Putting rocks in the bottom of a planter box
Large rocks or bricks can be used to fill the bottom to take up space but always cover them with some river sand. Putting small pebbles or rocks in the bottom of the planter box can create an area for water collect rather than drain away.
It is better to use a coarse sand on the bottom later of a planter box rather than small rocks. If this does get mixed through with your soil it will add structure and stop the soil level from dropping as quickly.
Ingredients to fill a planter box
River sand is great to layer on the bottom layer of a deeper planter box. If you have a large planter box this will save you money as it is more affordable than planter box soil mix. Make sure there is at least 12-18 inches of topsoil for growing vegetables.
River sand has larger pieces that regular fine sand used in sandpits so will be great for drainage.
Good quality raised garden mix
Use good quality raised garden mix as the main ingredient to fill your planter box. Raised garden mix will contain a mix of organic matter and sand which is perfect for raised garden beds. It will resist compaction and improve drainage for your plants.
This will be available from your local garden center, hardware store or from landscaping suppliers. Raised planter box mixes can also be bought online, check out this one from Amazon.
Adding extra compost to your planter box will add organic matter and soil microbes. If you have home made compost this will be the best as it will contain a range of materials from your own home and will be completely organic. You can also add worm castings from your worm farm.
Mix the compost through the top layer of your planter box so that the plant roots can reach it. You can add more compost to the top of the container over time as your plants go or when you replace vegetables after the growing season.
Aged cow or chicken manure
Aged cow or chicken manure is the perfect ingredient to add to your planter box. It is a mild fertilizer and will add organic matter. This will help to fill out your container and your plants will love it. Add around 1 shovel full per square foot and dig it through.
After adding the aged cow or chicken manure you can plant straight into the soil. Leave the bed for 2 weeks if you can to let it settle in and then surround the plants with mulch.
What to put in the bottom of a planter box
If you have a deeper planter box, you can use river sand as a base to fill the bottom layer. River sand will drain well and will save you money if you have a large planter box to fill. Make sure that your plants still have a good depth of good quality soil to grow in.
If you have a smaller planter box then fill the whole depth with planter box mix. Having at least 2-3 feet of good quality soil mix will give vegetables the depth they need to grow well. This is also important if you are planting a deep rooted plant like small trees. Ideally trees like citrus will need at least 3 feet of soil depth to thrive over time.
How to fill large planter boxes
To fill large, deep planter boxes you can fill the bottom with organic materials including logs, sticks and plant matter. This will work well if you vegetable garden is away from your house as there is risk of termites. This process is based on a growing method similar to Hugelkultur beds.
For Hugelkultur beds, logs are buried in the ground and covered with organic matter and garden soil. The logs will break down over time improving the soil. These logs can take 10 or more years to break down completely.
If logs are placed at the bottom of garden beds, they will hold the soil up for a long time and gradually break down releasing organic matter.
You can also use river sand or a cheaper potting soil mix and improve it with organic matter, river sand and worm castings. Landscaping suppliers can deliver larger amounts which will be a much cheaper way to fill your container.
Replacing soil in planter boxes
You will not need to replace soil in your planter box, instead top it up with compost, more raised soil mix and aged cow manure. Over time the soil level will drop as it compacts with gravity. Mix through more organic matter with a fork to lighten the soil and give your plants more nutrients.
If you aren’t replacing the plants, you can add compost around the root zone on top of the soil without disturbing the roots.
How to fill a planter box – Summary
Filling a planter box successfully comes down to finding the right mix of soil that has a good mix of organic matter and soil components like sand. Planter boxes are the perfect place to grow vegetables if you set them up right.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.