Grow bags are a convenient solution for gardeners who are short on space, but every gardener can enjoy the benefits of grow bags. In this week’s episode, I answer all your questions on gardening in grow bags, including all the advantages as well as the challenges and how to overcome them. We polled our social media followers recently on what they wanted to know about grow bags and boy did we get a lot of questions. Answers to all of them are in this podcast and show notes.
Even though I have a big garden with 16 really large raised beds, I have added grow bags to my setup. They are useful and convenient for a number of reasons that I will expand on in this post. I always like to experiment in the garden beyond what’s growing in my raised beds. Grow bags are like pop-up mini raised gardens that allow me to temporarily add growing space. When I don’t need that space anymore, I can empty the bags and put them in storage.
Though grow bags are useful for every gardener, it’s really the urban gardeners and small-space gardeners that have the most to gain. Grow bags are perfect for gardening on a deck, patio or stoop.
Joining me on the podcast is Amy Prentice, the Director of Marketing and Communications here at Agrivana Media®. Amy used grow bags herself for the first time this growing season and shares frequently asked questions about grow bag best practices.
Before proceeding, I want to let you know that my 2021 Holiday Gift Guide is out now. You can find 20 great gift ideas for the gardeners in your life at joegardener.com/giftguide.
Advantages of Grow Bags
Extra Space When You Need It: When I plant a crop in my raised bed garden that takes a while to grow, it limits my opportunities for succession plantings. I can create more opportunities in my garden by adding grow bags. Grow bags come in various sizes, so you can find ones that are large enough for anything you grow.
Easy Harvesting: One of my favorite attributes of grow bags is their convenience when harvesting root crops. If you are harvesting sweet potatoes or new potatoes, you don’t have to dig around with a garden fork, potentially damaging the tubers. I pour the bags out over a sifter, so 100 percent of what was growing underground remains intact and the soil can be saved and reused. Some grow bags have flaps on the sides so you can take only the tubers you need for dinner that night.
Attract Pollinators: To attract pollinators to your garden and add some color, you can scatter around grow bags and plant flowers in them. If you need to move the bags to a better location, you can lift them by their handles or put them on rollers.
Crop Rotation: Because I grow many crops that are in the same family, it’s challenging to practice crop rotation in my 16 raised beds. Crop families should be on a four-year rotation schedule, moving from one bed to another annually before returning to their original bed. I have built up soil-borne pathogens in my raised beds. If you don’t rotate often enough, it’s not a matter of if it will happen, but when. Now I am forced to find new places to grow tomatoes and peppers (both in the nightshade family) and grow bags are a perfect solution. I don’t have to go through the expense of building more beds. Wood’s expensive right now, and grow bags are very inexpensive, relatively speaking. I fill them up with clean soil that doesn’t have years of pathogen build-up.
Amy can also attest to the benefits of using grow bags with clean soil. After struggling with tomato and pepper diseases and poor growth in her beds, she tried using grow bags with potting soil. She reports the plants were bushier with more fruit set and fewer disease issues. In addition to being free of pathogens, the clean soil may have also had more nutrients than the soil in her beds. The experience attuned Amy to issues in her garden and she says it made her aware that she has some work to do on her garden soil.
Grow Bags Vs. Plastic Containers
Podcast listener Belinda wanted to know if she should use grow bags when she already has success with plastic containers. I subscribe to the idea that if it ain’t broke, don’t fix it. However, you may find that grow bags get even better results.
One of the differences between a plant grown in a rigid plastic container and a plant grown in a grow bag is how the roots behave. In a plastic container, when the roots get to the edge, they will continue to grow but will be forced to grow in a circular pattern. This will cause the plants to become what’s known as “root bound” or “pot bound,” which restricts their access to water and nutrients. For a plant that stays in a pot for a long time, like a shrub, this condition can eventually kill it as the plant girdles itself. This is not as much of a risk for annuals or seasonal crops.
When the roots reach the edge of a grow bag, they stop growing. That’s because the bags are porous, allowing air to get through. When the roots reach air, they stop growing in that direction. This is known as “air pruning.” At the same time, roots need air on the soil to grow, so the air circulation through the walls of grow bags leads to healthier roots.
Choosing the Best Grow Bags
The best material for grow bags is heavy-duty landscape fabric, which is typically made from polypropylene. The material is typically food-grade, free of anything harmful and long-lasting. Another common landscape fabric material is polyethylene, which is basically recycled plastic bottles. No matter the material, the common denominator is it is spun-bond fabric, which is proven to have the best staying power.
You get what you pay for. If you look online and shop based on price alone, you won’t get the most durable bags. Some bags don’t have handles, which are very important for the bags’ mobility, though also keep in mind that not all handles are equal. Amy found that on cheaper grow bags the handles fell apart before the end of one season. She recommends looking at bags that have handles that are part of the bag, not just sewn onto the outside.
Look at the reputation of the manufacturer and seller. I like Root Pouch, Smart Pots and Spring Pots. You can’t go wrong with those brands, but you can always shop around, experiment and find a brand you like.
UV light, water, heat and other environmental conditions are going to work to break down whatever material you are using. You can make grow bags yourself out of burlap or cotton, but consider their longevity, durability and strength. If you will lift the bags up when they are full and move them around, they will need sturdy handles and extra strong seams.
Black Vs. Colorful Grow Bags
Mark in my Beginning Gardener Fundamentals course wonders if the color of grow bags makes a difference. Black is the easier color to come by, but there are other options, some that blend in well with the garden and others in bright green or blue.
The main concern with colored grow bags is how much light they allow through. Roots don’t want to be exposed to light, so avoid light-colored, thin bags. But if the material is thick enough, the color of the bag won’t matter.
I prefer darker-colored bags because they absorb more heat. The retained heat will protect the roots on cold nights and lead to more productive plants.
The Right Size Grow Bags
Many listeners were interested in what size I recommend for grow bags from one crop to the next. It’s not just the volume of the bags that matters but the dimensions too. Some bags are deeper than they are wide, and vice versa.
For shallow-rooted plants like onions, garlic and lettuce, you can use a wide and shallow grow bag. But for deep-rooted plants like carrots, parsnips, tomatoes, peppers and Brussels sprouts, a taller bag is needed.
Aside from being conscious of the height and width, I choose bags that have at least 5 gallons of volume. The smaller sizes don’t have enough room for adequate root mass. I always want to give my plants all the opportunities to grow. If they want to grow a huge root system out and down to support the top growth, then I’ll provide the room for them to do that.
Allow 5 gallons per big plant, at a minimum. However, I wouldn’t grow two indeterminate tomatoes in the same 10-gallon bag or even in the same 15-gallon bag. If you put a plant with an expansive root system in a larger grow bag, the root will grow that much larger and compete with plants in the same bag. Instead of two tomatoes in the same large bag, you can mix a tomato with a shallow-rooted crop.
There are budget concerns when using larger grow bags. Not only are larger bags pricier, but it also costs more to fill them with good quality soil. Factor that into your buying decision. And if growing just one cabbage or head of lettuce or a few sprigs of spinach, you can get away with a smaller bag.
Amy had a 25-gallon grow bag, which at that size is more like a medium raised bed. She planted a tomato in the middle and onions around the side, which was an efficient use of the space. But the downside is the cost of filling it up with soil. Plus, once a bag of that size is in place and full, it’s very challenging to move it. A 25-gallon grow bag is a good option for a small fruit tree that gets moved indoors in winter, or for sweet potatoes, which are more productive with larger tubers when given the room.
What Can Grow in a Grow Bag
Many listeners were curious about what plants are best suited to grow bags. The answer is that you can grow nearly any crop in a grow bag that you can grow in a container or raised bed. If you put the plants in the right environmental conditions with well-drained soil and apply water it properly, the plants will perform well. That said, I recommend plants that grow to maturity quickly. There is less risk over a shorter period of growing time.
What to Place Grow Bags On
Students from my Master Seed Starting course wanted to know what surface to place grow bags on. Sherry noted that her tomatoes in bags placed on her deck did not do as well as tomatoes in bags placed directly on her lawn.
The surface does make a difference. On a hard surface, the grow bags only received the water from rain and from Sherry. But the bags sitting on the lawn also had the opportunity to wick up moisture from the bottom.
Another issue with a solid base like a driveway or patio is staining. The bag will leave a dirty ring, but you can put a liner underneath the bag to protect the surface. You could also prop the bag up on bricks, but then the bottom of the bag will be exposed and will dry out the root mass faster. You’ll need to water more often.
Spacing Plants When Using Grow Bags
Most grow bags are cylindrical while — Cindy from the joegardener Facebook group points out — most spacing guidance for plants is based on a square.
To give plants adequate space within a grow bag, I eyeball the square within the circle. That leaves four semi-circles on the periphery where more plants could potentially fit.
What Soil Should Be Used in Grow Bags
Grow bags require well-draining soil that retains just enough moisture so the root mass will not dry out between waterings. Bagged products labeled as potting soil or container mix are ideal.
Many potting soils contain peat moss but more and more mixes are now using coir (coconut fiber) instead. And then there is perlite, those white beads that look like pieces of styrofoam. Perlite is a mineral that retains moisture and makes potting mix lighter. Alternatively, the mix may contain vermiculite, which is flaky and serves the same purpose as perlite.
Bagged mixes may also contain wood fines or forest products, which is just finely ground wood.
You don’t have to use new soil each year unless soil-borne diseases have made their way into your grow bag. Research the diseases that occurred on the plants. You’ll discover if the pathogens are soil-borne. Or you can have the soil tested for pathogens at your local extension service.
To use clean soil again another year, you can refresh it by dumping the soil out into a wheelbarrow or large container that it can expand in. Pick out any clumps or woody roots then add compost. The additional compost should amount to about 25 percent of the original volume of the soil. The compost will provide the nutrients and microbiology that plants need to succeed. You can also add slow-release organic fertilizer.
I’m often asked about planting in straight compost. While you can get great results in compost, it doesn’t have everything that plants need. Even though compost is quite diverse on its own, plants require diverse minerals and nutrients that aren’t found in great numbers in straight compost. Your compost can be amended with lime, wood fines and perlite/vermiculite to produce a better container mix.
Adding Fertilizer to Grow Bags
Nutrients and supplemental fertilizer will leach out of grow bags over time, but there are things you can do to alleviate those concerns.
I am vigilant about making the rounds regularly with organic liquid fertilizer, whether it’s a soluble product — one that comes dry by dissolves in water — or it sold in liquid form and then diluted, like fish emulsion. Between the supplemental fertilizer that I apply and the compost bound to the soil, the plants have the nutrients they need.
Watering Grow Bags
A downside of grow bags is they dry out fast. It is hard to keep up unless you automate your watering with a drip irrigation system. These systems just click together — assembly is a no-brainer. Then you get an automatic battery-power timer. It’s a worry-free way to make sure your plants get the water they need.
To help the soil retain moisture between waterings, apply 2 inches of organic mulch, such as shredded leaves or straw. Mulch will also suppress weeds, and it provides valuable nutrients and organic matter to the soil and plants as it breaks down.
Grow Bags and Sunlight
Vegetable crops require full sun: between six and eight hours a day of direct sunlight. Some gardeners move their container gardens or grow bags around each day to chase the sun and get out of the shade.
Cindy from the joegardener Facebook group moves her grow bags around on platforms with wheels, but she can’t always keep up with this demanding task. Fortunately, plants don’t need us to be perfect. In nature, there are cloudy days, so your plants can survive being left in the shade for half a day once or twice a week. Just do the best you can and don’t lose any sleep over it.
Grow Bags and Pests
Grow bags don’t have any more pest issues than raised beds do. In fact, there may be fewer pest issues because it is harder for pets to get in. If you do find pests, follow the same integrated pest management practices like you would anywhere else in the garden.
Some grow bags have flaps on the side that you can open to check on or harvest root crops. Occasionally you may open a flap and discover ants or other insects are inside the bag.
Ants aren’t really a problem in the garden (unless they are fire ants). Run-of-the-mill ants aerate the soil and really don’t pose a threat. I don’t recommend intervening to control ants.
Using Supports in Grow Bags
When growing plants that require supports, don’t overthink it. If a tomato cage risks puncturing holes in the bottom of a grow bag, don’t worry about it. The bags are porous anyway, so a few holes won’t affect their integrity.
You can also use a cage that is bigger than the bag itself, but you may need to custom make a cage, such as a concrete reinforcing wire cage, to achieve that size. You can also pound stakes into the ground around the outside of the bag.
I set grow bags along a fence and added livestock panels. The panels provided plenty of height and I could train tomato vines espalier style.
What to Do with Grow Bags in Winter
Before the winter months, you can empty your grow bags of soil, sanitize them, fold them flat and put them in storage until spring. But you can leave them full of soil and in place for the winter if you choose.
A few crops can be overwintered in grow bags. Annette from Beginning Gardener Fundamentals plans to leave her potatoes out in grow bags over the winter for storage so she can take potatoes out as she needs them. Carrots also store really well in cold weather and could be left outside as well.
Overwintering in grow bags is challenging for most crops. The entire root mass is above ground level, so the roots are more exposed to cold. Overwintered crops would be better suited to in-ground or raised-bed gardening.
If you do plan on putting empty bags into storage over the winter, give them a good rinse off with a hose first and dip them in soapy water. You can even put them in your washing machine with cold water, but not the dryer. Always allow them to air dry. Once they are completely dry, they can be packed away.
I hope this episode addressed the questions you have about gardening in grow bags. If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.
What other questions do you have about gardening in grow bags? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Smart Pots – 15-gallon
Smart Pots – 15-gallon five pack
Smart Pots – 20-gallon five pack
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.