How to Grow Tomatoes in Grow Bags

I’ve always been split on tomato grow bags. They’re undoubtedly useful, and they can even speed up
your harvest, but they’re also expensive and covered in single-use plastic. With that in mind, we
wanted to take a balanced look at tomato grow bags so you, our readers, can decide what’s right for
you.

Tomato grow bags are convenient, but what do they really do? We’ve tested a few different tomato grow bags to compare the success rate of different composts, and tried a couple of innovative growing methods too.

Read on for everything you need to know about tomato grow bags, and whether they’re really worth the money.

What are tomato grow bags?

Tomato grow bags are pre-prepared compost bags, filled with ready-mixed compost and tomato food to help young plants establish roots, and provide controlled nutrients for the early parts of the season.

Tomato grow bags are sold with markings to guide users on planting tomatoes, provide appropriate spacing, and sometimes guide markings for drainage holes. They are marketed as a hassle-free way to grow tomatoes in a limited space and are great for beginners.

What’s in tomato grow bags?

Tomato grow bags contain a mix of basic ingredients, making them pretty easy to recreate at home on a budget, but there are some components to most grow bags that are being gradually phased out by EU, US and UK governments in an effort to limit ecological and climate damage.

Most tomato grow bags contain:

Organic matter (50% compost, 50% peat)
Nitrogen-rich fertilizer (slow-release granules or liquid fertilizer infused into the compost mix)

Tomato grow bags, pros and cons

There are a range of benefits to tomato grow bags, but is important to weigh them up against alternatives. For example, tomato grow bags help to warm soil so you can plant tomatoes out slightly earlier than planting in the ground. Alternatively, filling re-usable black plastic pots with compost has the same effect.

Similarly, tomato grow bags help to retain moisture, and are best used in conjunction with grow trays, which allow water to be reabsorbed later. The same results can be attained with any container used with grow trays.

	PROS	CONS
Cost	Save money on early feeding, as most are infused with fertilizer already.	More expensive than standard compost (usually around double
Success rate	Same success rate as any other method if correctly watered and fed regularly.
Drainage	Grow bags hold moisture well and allow excess water out.	Tomato grow bags have a habit of holding too much water and stagnating when placed in the shade.
Fertility	Tomato grow bags are usually infused with tomato fertiliser, or similar organic feeds to help seedlings get a healthy start.	Tomatoes grown from seed will need potting on into small pots while roots establish. Planting mall seedlings straight into grow bags can cause over-feeding.
Ease	Grow bags are incredibly easy to use.	N/A
Materials	N/A	Tomato grow bags use more single-use plastic, and are generally sold with 50% peat content (there are brands that sell peat free grow bags – look for those)

How to use a tomato grow bag (three ways)

There are three grow bag methods that are generally accepted as effective, but some of definitely more efficient than others, and one, in particular, stands out as producing distinctly higher yields on healthier plants.

Method 1. Standard

The normal way to use a tomato grow bag is to lay it horizontally on the ground and plant straight into the bag using the markings on the container as a guide. It is important to shake your grow bag before opening it to loosen the compost, which is usually heavily compacted.

Place a grow tray on the floor, and set up any supports you plan to use before you begin.
Shake, hit and fold your grow bag until the soil inside feels loose.
Lay the grow bag on the tray, then pierce drainage holes along the side.
Cut out the marked squares on your grow bag.
Plant tomato seedlings straight into the compost.
Water well.

Method 2. With pots

In recent years, we’ve been inserting pots with their bases cut out into tomato grow bags, half filling them with compost, and planting into the pots with grow bags underneath. We’ve found this method to produce more fruit, and also limit disease. For vining tomatoes, in particular, this is the best way to use grow bags.

Follow steps 1-4 in the normal method above.
Cut the base out of three black plastic pots (leave an inch around the edge so the pot has some structure).
Twist the pots into the grow bag compost, so they are firmly in place.
Half fill the black plastic pots with standard peat-free compost.
Plant tomatoes into the pots.
Water enough to saturate the top pot, but not so much that its runs out of the grow bag.

Why this works: Tomato grow bags are infused with fertilizer, which can stunt young tomato plants, rather than help them. When you plant into unfertilized compost, your tomatoes grow stronger roots, which are well developed by the time they reach the grow bag.
By leaving space in each pot, you can water generously every day, without wetting the leaves, or splashing water onto the grow bag. This gives each plant a dedicated water supply, but excess keeps the grow bag evenly moist.

Method 3. Bag planter

Using grow bags as pots is definitely on trend right now, with dozens of magazines recommending it in 2022, but does it actually do anything? Vining tomatoes need plenty of root space, so it’s generally advised to plant two to a bag, rather than three. By literally cutting your grow bag in half, rolling over the edges and placing it with each end facing the ground you create a vertical grow bag that acts like a pot for any plant.

The vertical method allows water to drain downwards, so young tomatoes have to send roots in search of water, rooting down, rather than out.

Really, the only practical reason to use grow bags like this is to save space. One grow bag horizontally on the ground takes up about 1m of ground. The same grow bag split in half and used upright takes about half the space. It also allows tomatoes to be placed further apart if needed, or separated if you spot signs of blight.

How many tomatoes to plant per grow bag

Using the standard method (laying grow bags on the ground, and planting into cut holes) it’s possible to grow three of any variety of tomato in each bag, but there are some caveats to that.

Bush tomatoes are dense but have smaller root runs than vining tomatoes. So, when using grow bags to plant bush tomatoes, you can easily fit three plants per grow bag. However, the dense foliage can lead to fungal problems as moisture sits on their leaves. To avoid this, place your grow bags somewhere with great ventilation.

Cordon tomatoes can be planted three to a bag, but will compete for nutrients. For most cordon varieties (but especially beefsteak and salad tomatoes) it is better to plant two tomato plants per grow bag and ignore the instructions on the bag. Plant vining tomatoes so they have half a bag each to give them adequate root space and minimise competition.

Caring for tomatoes in grow bags

Before planting tomatoes, you need to do a few basic things to prepare your grow bags. After planting, you can treat them quite normally, but there are some simple things to look out for to make the most of your grow bags and avoid common mistakes.

Drainage holes for tomato grow bags

Grow bags need drainage holes. The leading brands tend to have small perforations along the base which are supposed to provide enough drainage, but in our experience, relying on these can lead to water logging.

In any tomato grow bag, pierce four holes per plant in the side of the grow bag. Piercing the base can create rapid drainage, and you’ll just waste half your water. Piercing two holes, on either side of the bag allows for a shallow reservoir in the bag, while excess water will run out through the sides.

How to support tomatoes in grow bags

Vining tomatoes in grow bags need support, but the shallow soil can make it difficult to bury canes firmly enough to hold the weight of growing vines.

There are a few options for supporting tomatoes in grow bags, but below, we’ve got two simple guides to our favourite methods:

Create a simple A-frame. A-frames are easy to build and hold much more weight than you might think. The triangular structures distribute weight and allow for stringing to give extra support. This method is best for heavy cropping varieties, where weight is a consideration.
Washing line. Our favourite method for supporting tomatoes in grow bags is to place them beneath a washing line. The strong line can take a lot of weight, and by tying a single twine, wrapped around the main stem of each plant to the cord above, you create simple support that doesn’t limit the growth of your plants. This method is best for airflow – read about string trellising your tomatoes in this article to learn more.

How to water tomatoes in grow bags

For one grow bag, you should use one small watering can (3-6L) each day, unless the soil remains wet, in which case, skip a day.

One benefit of tomato grow bags is that they encourage lateral rooting. Tomatoes grown in pots spiral their roots around the outside of their container in an attempt to search for nutrients. In grow bags, tomato roots are encouraged outwards; producing more fine roots that can take up water more efficiently.

Can you re-use tomato grow bags?

Tomato grow bags can be reused, but you will need to either top up the compost or change it entirely if you want to use them for tomatoes next year. Instead, use them to grow lower nutrient crops than thrive in poor soil with good drainage.

Carrots love spent compost, as their main concern is drainage, and an easy root run. Fill up pots with the used tomato compost and sow carrots directly. Or, use the spent grow bag to fill hanging baskets. Line the baskets with the plastic bag (black facing outwards) and fill them with a mix of used compost and new compost.

Good garden centers accept used black plastic in recycling bins so you can avoid dumping used single-use compost bags in your trash.

Tomato grow bag recommendations

You can pick up tomato grow bags at most good DIY stores and garden centers, but if not, there are some great composts available online to make your own. To use standard compost bags as grow bags, you’ll need to plant just one or two tomatoes per bag (allow 1 foot between plants).

Best peat-free tomato compost:

Blue Ribbon Organics produces natural garden products with a low climate impact. Their vegetable compost is made up of garden waste, grass clippings, leaves, rotted sod, vegetable waste, and wood chips. The results are a highly moisture retentive nitrogen-rich compost that is perfect for seedlings as well as adult plants. While the bags contain 1 cubic foot of compost, they can take two tomato plants.

Best tomato compost:

While expensive, these small tomato grow bags from Coast of Maine are packed with enough nutrients for a long growing season. These small bags are just enough for one tomato plant and contain a mix of rotted manure, peat moss, and ground crab shells which add a great calcium boost for later in the season to support healthy fruit.

Conclusion

I’ve got a love-hate relationship with grow bags, but there are some distinct advantages to using them – particularly when you’re running low on space. Whether you opt to use grow bags in the standard way or try something a little different, they are a brilliant way to get started with tomatoes.

Even seasoned growers still swear by grow bags as a fool-proof method for lower-maintenance tomato plants. And in our changing climate, anything that helps to minimize watering in the garden has to be a good thing!