Growing Dahlias in Grow Bags
I grow around 2000 dahlias which are planted in the ground. I have a few raised beds also. Of course, there is never enough room for all the dahlias. Grow bags are a quick, easy way to add more space. I grow around 30 to 40 dahlias in grow bags each year that I spread throughout the front on my dahlia rows in my dahlia field. Growing dahlias in grow bags is a great option if you do not have a lot of growing space.
Grow bags are fabric-like bags that can be filled with a growing medium and used to grow plants and vegetables. They are light enough to move around. You can easily lift them with their handles as they are light enough to move from one spot to another spot. You could also put them on rollers like a movable plant stand. The fabric grow bags have breathable fabric which allows more oxygen to plants’ roots to help with growth. Grow bags are different than a nursery pot as the porous fabric allows water to drain out the sides and bottom. Grow bags are perfect for a deck or patio; however, I find them perfect to use at the front of my dahlia rows. I like the black grow bags as they blend well with my dahlia field. I tried the beige colored bags, but they did discolor at the bottom by the middle of the season. Grow bags come in various sizes like 5 gallon, 7 gallon, 10 gallon, etc. I found I like the 7-gallon size the best as I could move it around if necessary.
I purchase my grow bags online at Amazon. I really like the Gardzen 7 gallon grow bags that come in a six pack for around $17.00 so they are only $2.83 each. I purchase them in a twenty pack so the price drops to $1.65 each. I can get two growing seasons out of each bag usually as I empty the bags in late fall and store them over the winter. They are reasonably priced. There are many different brands of grow bags online and many different sizes so you will need to research to see what will work best for you. Below is the link for the Gardzen grow bags. (I may earn a small commission over time from qualifying purchases as an Amazon Associate.)
I have also used the 1 gallon grow bags for starting dahlia tubers. I really like them when I am going to be grow a dahlia cutting for a longer period of time since the grow bags allows more oxygen to plant roots to help with growth. I had success with the 1 gallon grow bags when I was losing a dahlia that I planted due to rot. I dug up the dahlia and planted it in a 1 gallon grow bag.
In 2024, the dahlias growing in grow bags did so well. I was so happy. In 2023, I was so frustrated with grow bags. The dahlias did not grow and produce a lot of blooms. When they did produce a bloom, it was so small. They did not produce tubers. I posted about this throughout the 2023 growing season on my Instagram stories and other dahlia growers were telling me that they were having the same problem.
I asked my friend Lori, who has Broad River Blooms in Georgia, how she does it as she grows thousands of dahlias in grow bags. She recommended that I fertilize the grow bags about every 3 weeks and don’t forget to keep them watered during our hot summers. Grow bags can dry out fast during the summer. To find out more about growing dahlias in grow bags, you can go to Broad River Blooms website at https://www.broadriverblooms.com/ and subscribe to Lori's newsletter.
I also researched potting soil versus flower and vegetable garden soil. I really did not see much difference when I looked at the mixture in each bag. Apparently, there are differences. I had bags of flower and vegetable garden soil on hand in 2023, so I used that in my grow bags. At the end of the year, I felt the bagged garden soil was too heavy as my dahlias did not grow and the mixture stayed moist in the grow bags. I remembered adding perlite to some of the grow bags also and that did not seem to make a difference for some reason. I do not grow too many flowers in pots, just some annual flowers. I should have realized that the flower and vegetable garden soil was too heavy for grow bags. I should have used a potting mixture as it is designed to drain well and is designed for container plants. The ingredients of this flower and vegetable garden soil were regionally formulated from materials (derived from one or more following: reed-sedge peat, recycled forest products and/or composted rice hulls), sphagnum peat moss, horticultural perlite, ground dolomitic limestone, and a wetting agent. It all sounded ok as it had perlite to lighten the mixture.
In 2024, I used Miracle Gro Potting mix as that is what my local hardware and garden center carried in stock. The potting mix bag said it was formulated from (one or more of the following: processed forest products, peat, coir, and/or compost), and sphagnum peat moss, perlite, fertilizer, and a wetting agent. When I planted the dahlia tuber in the 7 gallon grow bag, I added a handful of compost.
Grow bags are great as the fabric material acts as barrier to stop voles from digging into the soil to reach the dahlia tubers. You do want your grow bags to be a strong fabric that can prevent voles from chewing a hole into the grow bag. Also, having a strong fabric grow bag is important so it can withstand the weight of your dahlia plant so you can move the bag if necessary.
In 2024, I tried Permatill to help deal with voles coming into my dahlia field and eating my dahlia tubers. Permatill is a brand name for a type of expanded slate soil amendment. Expanded slate is a lightweight and porous material made from natural slate that undergoes a process of expansion at high temperatures. Permatill is used in gardening and landscaping to improve soil structure, aeration, and drainage. Stalite Environmental in Salisbury, NC produces this product. I found it at a local gardening center and purchased several bags. Permatill also helps discourage voles because voles do not dig through materials that are coarse in texture. I mixed it into my soil as I planted dahlias and hopefully voles will not want to dig through my soil.
The reason I mentioned Permatill is because I did add a little Permatill to my grow bags as this expanded slate stores air, moisture, and soluble nutrients for my dahlias so I felt it would be beneficial. You can use Permatill has a mulch at the soil surface as it can help reduce soil borne diseases on the stems and foliage. Also, slugs do not like to crawl on Permatill. Voles can be persistent and can dig into the roots from the surface so this is extra protection. Also, the Permatill acts as a weed suppressant.
To recap, in 2024, I used a potting mixture, compost, and mixed in a little Permatill. I planted the dahlia tubers in a 7 gallon grow bag. I added some organic fertilizer on top and mixed it into the potting mixture. I use Plant-Tone by Espoma as it is easily available at my local farm and garden center. Espoma Plant-Tone (5-3-3) does have feather meal, poultry manure, bone meal, alfalfa meal, greensand, and Sulfate of Potash. I also put some Permatill on top as a deterrent for voles and a weed suppressant. I used Vigor Plant Booster every 3 weeks to fertilize the grow bags.
I use Vigor Plant Booster to fertilize my dahlias in my raised beds and dahlia field. I have used several different fish and seaweed fertilizers in the past and they all work great for using as a foliar or to add to your soil. For the last couple of years, I have been using Vigor Plant Booster. I follow Warren Vigor on Instagram. Warren is a past president of the Dahlia Society of Australia. Warren now lives in the U.S. in Michigan. Warren is a champion dahlia grower, and his dahlias really stand out at dahlia exhibition shows. He contributes his success to his plant booster. He has produced this soil conditioner that strengths plant growth. It contains NPK 9-2-6, Sulphur, calcium, magnesium, boron, copper, iron, manganese, zinc, selenium, cobalt, and carbon. It is made from cool processed whole fish, deep-water kelp, humic acid, fulvic acid, and sugar. I believe the sugar is molasses. For more information visit the Vigor Plant Booster website https://www.vigorfertizer.com.
Because the fabric grow bags are well aerated, they do dry out quickly once the dahlia has begun to produce flowers and has a root system. I had to water the established dahlias in the grow bags every other day or every day depending on how hot it was in the summer. You will want a water source near by for watering your grow bags. I have long hoses that I can use out in my dahlia field. Some dahlia growers use a drip system on a timer so you don’t have to worry about hand watering your grow bags. We do use a black poly tubing with 4 way assemblies and Jr Dripper with Stakes as a drip irrigation for some of my dahlia rows. For the grow bags that are near this area, we punched a hole into the black tubing and attached another dripper stake assembly for the grow bag. It is only going to water the grow bag when I water my dahlias in the field.
The dahlias in the grow bags cannot draw up nutrients from the soil like they can when planted in the ground. With the frequent watering, whatever nutrients that are in the grow bag mixture can leach out, so you do need to fertilize the grow bags more frequently than you do the dahlias that you grow in the ground. The potting mixture does have some slow-release fertilizer that helps along with the compost I mixed into the potting mixture. I also added the organic fertilizer on top that I mixed into the potting mixture. However, I still add a liquid fertilizer every 3 weeks.
By the end of the summer, you can barely see the grow bags in my dahlia field. The black grow bags blended so well in the front of my dahlia rows. I supported the dahlias with a 4-foot garden U-post that is positioned behind the grow bag. I tie the dahlias to the post about every 12 inches using twine. You could use tall tomato cages or rebar. You could also use a wooden stake in the ground right next to the grow bag. You can also put your grow bags along a fence and tie the dahlia to the fence as it grows.
Dahlias develop tubers when the day length is 12 hours or less in the early fall. I did continue to fertilize the grow bags through the beginning of September as it was very warm and humid in 2024. In a lot of U.S. locations, it was warm into November. I cut back the dahlias in the grow bags to around 12 inches tall in early November. I let them experience some patchy frost. We had a hard frost in late November so I did empty the tubers out of the grow bags prior to that hard frost. Dahlias need around 120 days to produce tubers. I did not plant dahlia tubers in the grow bags until the end of May so by October they had exceeded that amount of days for sure. The tubers looked great. I believe it was due to using the proper potting mixture and from watering and fertilizing often throughout the growing season.
So, no more “whatever I have on hand” soil for grow bags. I learned in 2024 that for my location and growing environment, my grow bags are going to need a good potting mixture with some compost mixed into the potting mixture, a water source nearby as you do need to water the grow bags often, and I need to use a slow-release fertilizer on top of the potting mixture. I also want to add a liquid fertilizer every 3 weeks as I feel that really did help a lot as I had such great tuber production. I know it probably sounds like a lot of fertilizing; however, remember the dahlias in the grow bags cannot draw up nutrients from the soil like they can when planted in the ground. With the frequent watering, whatever nutrients that are in the grow bag mixture can leach out so that is why it is good to fertilize more. You could probably make up your own potting mixture by mixing peat, vermiculite, and perlite together making sure it is a light, fluffy mixture. You could then add some compost and fertilizer to that mixture.
You also do not want to over crowd your grow bag. I usually plant one or two tubers of a dahlia cultivar per 7 gallon grow bags. I hope this article has helped you. Support your local home and garden centers and purchase whatever similar products you have available in your location. You may want to use a potting mixture though instead of flower & vegetable garden soil as a potting mixture does seem to be a lighter mixture. In 2025, I am following the same procedure. This year I have all 40 of my grow bags planted by May 20th. I did use a good potting mixture, a little perlite, a little PermaTill, and some compost. I am fertilizing with Espoma Plant Tone and Vigor Plant Booster. I cannot wait to see all the blooms this year.
UPDATE. My dahlias are doing great in 2025. So far we have had a wet summer so I have not had to water daily. I do stick my finger in the grow bag and check the moisture level. We had some high dew point days and water was not evaporating so the grow bags stayed wet. A few dahlias are starting to bloom. I have been giving them Vigor Plant Booster every 3 weeks.
I hope you are having fun growing dahlias this year!
Thank you for following along on my journey with dahlias.
Paula Fisher