Planter boxes for vegetables should be 12-18 inches deep. Planters 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 Depth | Vegetables, Herbs and Fruit |
| 12-18 Inches | Basil, parsley, chives, coriander, oregano Bok choy, garlic, corn, spinach, radish, shallots, onions, beetroot and lettuce. |
| 18-24 Inches | Rosemary Tomatoes, peppers, aubergine (eggplant), kale, and carrots |
| 24+ Inches | Potatoes, 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.
3 Key Factors to Determine the Perfect Depth for Your Vegetable Planter Box
When it comes to growing vegetables in planter boxes, the depth of the box is important. It will determine the root depth of the plants, the amount of soil needed, and the drainage of the box.
1. Root Depth
The depth of the planter box is directly related to the root depth of the plants you want to grow. Vegetables have different root depths, and it is important to choose a planter box that can accommodate the root depth of your vegetables.
For example, shallow-rooted vegetables like lettuce and herbs require a planter box that is 6 inches deep, while deep-rooted vegetables like parsnips and asparagus require a planter box that is at least 12 inches deep.
2. Drainage
Another reason why the depth of a planter box matters is because of drainage. A planter box that is too shallow can hinder drainage, leading to waterlogged soil and root rot.
On the other hand, a planter box that is too deep can lead to poor drainage, as water may not be able to reach the bottom of the box. The ideal depth for a planter box is 8-12 inches, as this provides sufficient drainage for most vegetables.
3. What you will be growing
Choosing the right depth for your planter box depends on the type of vegetables you want to grow. As mentioned earlier, shallow-rooted vegetables like lettuce and herbs require a planter box that is 6 inches deep, while deep-rooted vegetables like parsnips and asparagus require a planter box that is at least 12 inches deep.
Does my planter box need a bottom?
I 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.
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.
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.
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.
1. Use a good quality soil 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.
2. Keep the water up to your vegetables
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.
3. Feed them slow release or liquid fertilizer
After 3 months it is important to feed your vegetables with slow release or liquid fertilizer. I like to use pelleted chicken manure which will last for around 3 months and release nutrients slowly. If you use liquid fertilizer, add this every 4 weeks in spring.
Summary
The ideal depth for a planter box for vegetables is 12-18 inches, as this provides enough space for root development and to hold water, while also allowing for good drainage and aeration.
Happy growing.
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.