Introduction

There are dozens of growing mediums available for microgreen production. Each medium, or substrate, has its pros and cons. Some are cleaner, while others make a mess. Some are reusable, while others are single-use. Some require a lot of space and equipment, while others are compact. Before selecting a growing medium, it’s important to consider the context of your situation. Are you growing for personal consumption or profit? Do you want to stay small or scale as fast as possible? Will you want to grow dozens of varieties with varied needs or a select few? This article covers the basic questions you should ask before selecting a substrate. We provide a list of the most common substrates used in the industry and then conclude with our suggestions. It’s important to remember that you can always change substrates later on. However, microgreens grow and behave differently based on the substrate. For example, we grew broccoli microgreens using Promix and ReNu Terra grow mats. The ReNu Terra broccoli takes four days longer than the Promix broccoli to reach the same size, all else being equal. I encourage you to experiment with growing many different varieties on various mediums before committing to one.

Contextual Questions

As much as people want to crown a winner, everything is contextual. The largest North American microgreen producers use soil and soil-like grow mediums. Therefore, those must be the best for microgreen production, right? Farming Turtles in Rhode Island grows for major retailers, like Whole Foods, and they grow in peat-based soil. City Roots in Columbia, SC, also grows for major East Coast retailers, like Sprouts Market. City Roots also grows in compost-based or peat-based soil. However, both farms have also been around longer than nearly anyone reading this article. They own large pieces of equipment capable of handling heavy, dirty soil. They have the land to house the equipment. They grow in greenhouses where getting soil and water on the floor is less problematic. The list goes on. When deciding what growing medium is best for you, ask yourself a few questions.

• How clean do you want to keep your farm or production area? If you grow in your carpeted living room, you might want a cleaner substrate, such as polyester or silicone mats. If you grow in a concrete basement or a greenhouse, cleanliness might not matter as much.
• How much space do you have? A stack of 65 hemp mats can fit in a small box. A bale of Promix will also fill 65 trays and take up the same space, but only while it’s compressed. The bale of Promix takes up space, but you need a bin to put it in as well. You’ll use the bin to break up the compressed bale to then scoop it into trays. You also might want a place outside where you can make a mess while filling trays with Promix. Farming with Promix will take up more space than farming with a grow mat.
• Do you want a reusable growing medium or a single-use medium? Reusable mediums tend to be cleaner than single-use mediums. They also leave less residue on the trays, which makes the trays easier to clean and sanitize. However, you’ll need a system for cleaning and sanitizing the substrate as well. For example, if you use Urban Greens' polyester mats, you must let the residue dry, scrape it away, sanitize, rinse, and dry the mats. That’s a whole cleaning system on top of cleaning the grow trays. Using a single-use medium like Coco Loco will leave your trays much dirtier. It will take more work to clean and sanitize the trays compared to a reusable medium. You also need to consider where the “spent” soil will go. Can you put it into a garden box? Is there a local composting company that will pick it up? In my opinion, you shouldn't reuse soil-based mediums, like Coco Loco. See the article linked below for more details.
• Do you plan on growing for profit or personal consumption? Nobody wants to spend money unnecessarily. Of course, everyone wants cheaper products and inputs, all else being equal. Everything isn’t equal, though. I’ve yet to test a grow mat or reusable medium that performs as well as soil-based mediums. The microgreens always germinate slower and yield less than soil-grown microgreens. If you're growing for profit, substrate cost and microgreen performance matter. If you’re growing for personal consumption, it’s not that critical. You won’t grow enough microgreens for the substrate’s cost and performance to make a huge difference. If the substrate works well, I suggest a cleaner, easier-to-handle medium.
• Is the medium suitable for all crops or only select varieties? Most microgreen varieties grow directly on top of the substrate. Most varieties don't need to be planted under the substrate or covered with anything, like soil or vermiculite. A handful of important varieties do need to be covered, though. Notably, cilantro, beet, and chard need to be top-dressed. Top dressing helps remove seed hulls during the germination process. I haven’t seen anyone successfully grow cilantro, beet, or chard using grow mats or reusable mediums. If you grow these popular but demanding varieties, consider their growing needs when choosing a medium.

The Contestants

Here is a non-exhaustive list of substrates I’ve encountered over the years. I have not tried at least half of these in my farm. Most for-profit farms I know that grow more than 200 trays per week use one of the soil substrates. The Cleanliness, Versatility, and Performance columns are my subjective rankings. As previously mentioned, soil substrates are messier, while reusable mediums are cleaner. Versatility means how many microgreen varieties will grow well with that substrate. For example, you can’t grow beet microgreens well without top-dressing the seeds. You can’t top dress using grow mats or reusable substrates. Performance covers germination speed, germination uniformity, growth rate, and yield. Cost is the cost to purchase one tray of that substrate. Of course, reusable substrates have a higher purchase price, but a cheaper per-use cost.

Type of Medium	Reusable?	Grow Medium	Cleanliness	Versatility	Cost	Performance
Soil: Peat-Based	No	Promix (BX	MP	HP)	Dirty	High
Soil: Peat-Based	No	Sunshine Mix #5	Dirty	High	Low	High
Soil: Peat-Based	No	Coast of Maine	Dirty	High	Low	High
Soil: Coco-Based	No	Coco Loco	Dirty	High	Low	High
Grow Mat	No	Hemp	Moderate	Low	Moderate	Moderate
Grow Mat	No	ReNu Terra	Moderate	Low	Moderate	Moderate
Grow Mat	No	Bamboo	Moderate	Low	Moderate	Moderate
Grow Mat	No	Coco Coir	Moderate	Low	Moderate	Moderate
Grow Mat	No	Biostrate	Minor	Low	Moderate	Moderate
Grow Mat	No	Burlap	Minor	Low	Moderate	Low
Grow Mat	No	Rockwool	Minor	Low	Moderate	Moderate
Grow Mat	No	Paper Towels	Clean	Low	Low	Low
Grow Mat	Yes	Polyester Mats	Clean	Low	High	Moderate
Grow Mat	Yes	Steel Mesh or Silicone	Clean	Low	High	Low
No Substrate	Yes	Mesh Bottom Trays	Clean	Low	High	Low

My Suggestions

If you plan on running a microgreen business, my suggestion is to use a soil-based medium. They're messier, require expensive equipment to use at scale, and take up more space. However, in my experience and from what I’ve seen of others, soil produces the best and highest-yielding trays of microgreens. If you plan to grow for personal consumption only, my suggestion is to stick to grow mats or reusable substrates. These mediums won’t produce the best microgreens, but they will still produce a quality product. Using soil for personal consumption is fine if you can put up with the extra mess and space requirements, but that’s a personal decision.

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