Grow bags are specialized fabric containers popular among home gardeners for their exceptional root aeration and drainage. The permeable fabric walls allow for “air-pruning,” encouraging a dense, fibrous root ball instead of root circling. For a large 20-gallon container, the first challenge is determining the exact volume of growing medium required. Calculating this volume prevents the costly mistake of purchasing too much or running short during planting.
Calculating Soil Volume for a 20-Gallon Bag
A US liquid gallon is equivalent to approximately 0.1337 cubic feet. Therefore, a 20-gallon grow bag requires about 2.67 cubic feet of growing medium to be fully filled. Since commercial potting mixes are commonly sold in cubic foot bags, plan on buying at least three bags to meet this volume requirement. A 20-gallon container also holds 80 quarts of material, as four quarts equal one gallon.
The final volume of soil actually used may be slightly less than the calculated 2.67 cubic feet. This is because the medium settles over time, especially after the initial watering. Additionally, most gardeners leave a few inches of space at the top of the bag to prevent water and soil from spilling out during irrigation.
Essential Components of a Quality Grow Bag Mix
The quality of the soil mix directly determines the health and productivity of plants in a confined space. Grow bags demand a specialized, “soilless” mix that supports robust root growth and manages moisture effectively. Standard garden soil is unsuitable because its high mineral content compacts easily within a container. This compaction restricts oxygen to the roots and impedes the necessary drainage that grow bags are designed to provide.
A successful grow bag medium must satisfy three requirements: aeration, water retention, and nutrition. Aeration is achieved by incorporating lightweight, inert materials such as perlite (expanded volcanic glass) or pumice. These components create non-compacting air pockets, ensuring oxygen is available for root respiration and preventing waterlogging.
Water retention is provided by organic materials like coco coir or peat moss. Coco coir is often favored for its neutral pH and ability to hold moisture while still allowing good drainage. Nutrition comes from rich organic matter like compost, worm castings, or a slow-release granular fertilizer. A general-purpose mixture that balances these needs might consist of roughly 40% water retention material, 30% compost, and 20% aeration material, with the remaining 10% being a base potting mix or other nutrient amendments.
Strategies for Cost-Effective Filling
A major consideration for a large 20-gallon bag is the high cost associated with purchasing 2.67 cubic feet of premium soilless mix. A cost-effective solution is to use bulky filler materials to occupy the lower, non-root-intensive space of the container, a technique inspired by Hugelkultur. The goal is to maximize the use of expensive potting mix in the top 10 to 12 inches where the plant’s active root zone will primarily reside.
This technique uses materials that are slow to decompose and are essentially free, such as:
- Wood chips
- Pinecones
- Inverted plastic nursery pots
- Crushed plastic bottles
Placing these items at the bottom of the bag significantly reduces the total volume of required potting soil, as the roots of most annual vegetables will not reach them in the first season. For example, filling the bottom third of the bag with filler can save almost a full cubic foot of premium mix.
A physical barrier, such as a layer of old landscape fabric or burlap, should be placed between the bulky filler and the specialty potting mix. This barrier prevents finer soil particles from filtering down and settling into the air pockets created by the filler. This ensures the drainage and aeration benefits are maintained while providing a stable, well-drained base.