This blog post is an edited form of an email I exchanged with a customer. Thanks to the customer who provoked the conversation- I’m sharing the advice I gave you to the rest of my readers! Also thanks to the "New Jersey" customer who got me thinking about garlic in pots during correspondences last year.
I haven't grown garlic in containers yet but should do so, so I can give more qualified advice. A customer in New Jersey has done it and recommended what I call "thrifty" plants- plants which don't put on much top growth (compared to, say, a Porcelain or Purple Stripe) but still yield decent bulb sizes. He had recommended Creoles.
Here, I'll put together two things I do know a fair amount about- the physics of temperature and root volume, and the physiology of garlic. This may help you brainstorm about how to do garlic in pots well, and will be the foundation for me trying it next winter, so I can gain some experience, too.
For me, container growing is basically about two things: Volume and depth of soil available for roots, and temperature swings.
Soil volume/depth
A garlic plant in the ground can send roots as deep as soil texture allows, and length of season. Sometimes, there are varieties which are never likely to get very deep, and so suffer less in containers. Porcelains and other big plants do need deep roots, and make for correspondingly big bulbs, all things being equal. I believe some studies say that Porcelains and other large types can go 12-24 inches deep depending on soil. I suspect thriftier types are essentially capped around 10-16 inches. Compacted soil, and/or shortened seasons, make these numbers go down. Containers are likely to a) have loose soils, and b) have long seasons (because they are likely to start earlier, more than anything), so the upper numbers should be considered.
I may guess that Artichoke, Turban, Asiatic, and Creole varieties may do well in containers. Maybe the Glazed family too. I'd expect middling results from Rocamboles and poor results from Porcelains, Purple Stripes, and Marbled Purple Stripes. If you are interested in new, unknown or wild types, I can answer you on a case by case basis since they're all quite different from each other.
Temperature swings
The second issue is temperature swings. A container is surrounded on all sides by ambient air temperatures. Conversely, the ground is a big "thermal battery"- meaning it can store and accumulate heat, and is also a source of lag since it takes time to heat back up after cooling. So, a container will get colder, sooner, in fall (i.e. shutting down root growth sooner in the season) but heat back up quicker in spring. This all changes, of course, if you cover containers with fabric or plastic, find ways to insulate them overwinter (making a wooden box, higher than your containers' height, putting them all together and covering with leaves could do this). I view the soil's thermal battery role as a safety feature, not a bug, so containers invite those more extreme swings. If you are skilled in managing other container plants, you already understand this and can adjust.
This really inspires me to finally experimentally do it myself this fall. I'm intrigued by small-space utilization and been working on a rack system in my (homestead sized) greenhouse. I may overwinter some in the greenhouse but also trial outdoor management of them.
Thriftier plants will soldier along better in the containers. My experimental focus will be on Artichoke, Turban, Asiatic, and Creole varieties, because I suspect they’ll do well. I’ll try at least one representative apiece, of Porcelains, Rocamboles, Glazed, Purple Stripes, Marbles, but I suspect they’ll do poorly.
PS- if you love math and want things expressed mathematically, out in the field, I typically judge that thrifty varieties need 25 square inches of space, and "space hogs" need 40 square inches- both for root volume and for their share of sunlight. If you extrapolate the average expected root depth- say, 10 or 12 inches for thrifty types, and 12-18 for space hogs, you can multiply and have the cubic inches needed for your containers:
250 cubic inches for thrifty types
400-600 ish for hogs.
One gallon is 231 cubic inches, so 1 gallon sized pots for thrifty types, or 1.5 to 2 gallon pots for space hogs.
Fertility
Field-planted crops have the advantage of nutrient translocation. All water-soluble nutrients are somewhat mobile within the water table of the soil, and roots absorbing water, will, by virtue of surface tension of water, continue to draw new water closer to them. This is why you can broadcast fertilizer somewhat haphazardly and it mostly gets around to everyone there, eventually. Underneath your feet, from a micro perspective, is a vast ocean of water, sloshing with soluble nutrients, that distribute themselves through the water over time. Areas high in soluble nutrients donate to areas low in soluble nutrients- it's the physics of coffee and tea. Given time,, things in solution will distribute themselves fairly evenly.
With containers, you suddenly don't have that. Over fertilize one container, and underfertilize another, and you'll burn one and starve the other. They're each a little micro-ocean, with few choices of donating or receiving excess nutrients.
Container fertility should be understood from that lens. You'll also have to be converting square-foot or acreage based rates to these little round pots. Those rates are already problematic because they may not respect variances in planting densities. Home gardeners often have garlic at 20-50,000 plants per acre, while commercial growers keep ratcheting up, often to 100,000 - 200,000 plants per acre. Anyways...for containers, you should just use the square footage value for simplicity, and assume that you could then cut the fertilizer bag's rate in half. My one gallon pots have a radius of 3.5 inches, which is 38 square inches of surface. That is 0.26 square feet.
Your fertilizer bag probably has at least a 1,000 square foot recommendation. I'd easily assume they have you throw double what you need to, in field growing. Take their recommended 1000 foot number, and assume it's actually enough for 2000 square feet of containers. That means...divide their poundage by 7,692. Or better yet, in ounce form, divide the number of recommended pounds by 480, and that is the ounce number for that one gallon container. If they say 50 lbs per 1000 square feet, that would be:
0.1 ounces per container.
If that sounds miniscule and difficult, the consolation is that this is actually to inform you of how much to spike your soil mix with. If you're growing, say, 50 containers of garlic, then you take 50 times 0.1...5 ounces of fertilizer mixed in those 50 gallons of soil. Not very much!