How Deep to Make a Planter Box for Vegetables | Easy Guide

Planter boxes for vegetables should be 12-18 inches deepPlanters that are 12 inches deep are great for growing vegetables like shallots, snow peas, onions, beetroot, and lettuce. A planter box that is 18 inches deep is great for vegetables like tomatoes, peppers, and carrots. Deeper planter boxes will allow healthy root growth and will hold water better than those that are shallow.

Planter Box DepthVegetables, Herbs and Fruit
12-18 InchesBasil, parsley, chives, coriander, oregano Bok choy, garlic, corn, spinach, radish, shallots, onions, beetroot and lettuce.
18-24 InchesRosemary Tomatoes, peppers, aubergine (eggplant), kale, and carrots
24+ InchesPotatoes, artichoke, asparagus, pumpkin, sweet potato, watermelon and rhubarb

Planter boxes are a great way to grow vegetables because they are great for drainage. I have 2 planter boxes at home that are full of vegetables.

Check out this Planter Box on Amazon.

How much soil to put in a planter box for vegetables

Fill the planter box with good quality soil leaving a 1-2 inch gap at the top so that soil will not be washed away as you water.  Over a period of months the soil in the planter box will shrink and compact.  Soil depth can go down by 1-2 inches by the end of the growing season. 

Top up the soil at the end of the growing season, in spring and fall.  Topping up the soil in the planter box twice per year will make sure the vegetables have enough room and nutrients to grow.

Vegetables and herbs that can grow in 6 inches of soil

Vegetables and herbs that can grow in 6 inches of soil include basil, parsley, chives, coriander, oregano Bok choy, garlic, corn, spinach, radish, shallots, onions, beetroot, rosemary and lettuce. 

TIP: The root system will usually mirror the height of the plant so if you plan to grow rosemary, you can usually get it to grow to the same height as the depth of your planter box.

I am growing basil in my shallow planter box.

Why planter box depth is important for vegetables

Planter box depth is important for vegetables to allow them to grow to their full size. Making sure there is enough room in the box to allow the roots of the vegetables to grow to a healthy size to get good produce.

If the plant can’t grow roots to their full size, the vegetables will be less productive. In a planter box that not deep enough, the vegetables will struggle to get the they need and won’t grow to their full potential.

If the roots are restricted you may see yellowing leaves which can be a symptom of not enough nitrogen getting to the plant which is a key nutrient needed for growth.

Does my planter box need a bottom?

I actually never put a bottom on my planter boxes.  Without a bottom the water can drain through and stop the soil from getting water-logged. If you can place your planter box on top of garden soil it will allow worms and good bacteria and fungi to come in and keep your planter box healthy.

If you have a really deep planter box, you can layer the first level with a coarse river sand which will aid with drainage. This is a good idea if you planter box is deeper than 20 inches.

How deep should a planter box be for tomatoes?

Tomatoes generally need a minimum of 12 inches of soil to grow.  It is even better to choose a deeper planter box up to 18 inches. Tomato plants like to send deep roots down to find water and nutrients. Giving your tomato plant the space to its roots down will give you a healthier plant and more tomatoes.

My tomatoes are growing in a planter box that is around 18 inches deep.

If your planter box sits on top of soil without a bottom, the can reach their roots down into the ground soil.  This is fine and allows the tomatoes a little extra space to spread their roots if they need it.

Tips for growing vegetables in planter boxes

Once you have worked out how deep you will be making your planter box, here are some great tips on growing vegetables successfully.

Use a great mix

Filling your planter box with a good soil mix is so important.  Choose a mix that is ready made for planter boxes at your local garden center or hardware store.

If you have a few planter boxes to fill you could have this delivered by your local landscape supplier, or pick it up yourself in a trailer. You can also mix in some perlite into the mix which will help your soil to hold water and air.

Avoid putting soil straight from your garden into your planter box.  It will compact over time and will not allow your vegetables to grow successfully.

Remember to keep the water up

Watering well is really important to produce a great crop of vegetables. Using a spray nozzle on your hose keep your soil most to get your veggies growing strong, particularly when they are small.

Just test the ground with your finger to see if it is moist.  If it is dry 1 inch below the soil, give your garden a good water. I recently bought a metal spray nozzle because it doesn’t leak and will last for years.  I have bought so many over my time but this has been my best investment so far.

Feed them with liquid fertilizer

I always like to use a liquid fertilizer in my raised garden beds.  I choose an organic liquid fertilizer and apply this monthly.  This has helped me to grow so many green vegetables, tomatoes and peppers in my raised garden beds.  This is a natural fertilizer so the plants can take up the nutrients they need.

I hope you have enjoyed this article about how deep a planter box needs to be to grow vegetables. Happy growing!