If you've ever found yourself looking for specific vegetable growing information, then I can almost guarantee that you have been frustrated, either from confusion around contradicting information or the lack of specifics. Take the garlic spacing recommendations from High Mowing Seeds as an example. They suggest planting hardneck garlic at 4-6 inches in row spacing and 18 - 24 inches between the rows. Okay. That could leave us with anything ranging from 72 square inches per bulb to 144 square inches per bulb. Is the ideal spacing for hardneck garlic really not more specific than that?
Usually, when I am presented with vague information like this, I look to the scientific research, but in this case, I have mostly come across studies relating to the spacing of softneck garlic varieties like this one:
From studies like this, it was obvious that spacing of garlic plants had a direct impact on the bulb size at harvest and the total yield of our harvest, but what spacing would give us the best results with hardneck garlic with our climate and growing conditions? It seemed like every grower was on a different page, and it was unclear whether they were basing their spacing decisions on science, tradition, or even just the size of one of their farm implements.
To give you an idea of the variance, here are a few examples of recommendations from well known vegetable growers. Pam Dawling suggests a minimum garlic spacing of 4.5 bulbs/ft² and a wider spacing of 2 bulbs/ft² for larger bulbs. John Jeavons suggests a high density spacing of 9 bulbs/ft². Eliot Coleman suggests a spacing of 3 bulbs/ft².
At times like this, we just need to a trial ourselves to find out the missing information, so this season we devoted some space to a garlic spacing trial. Read on for more information about the methods we used and what we learned.
For the last several years, we had been growing our garlic in our standard 30 inch wide beds in 4 rows 7.5 inches apart with an in row spacing of 6 inches. This spacing had been performing well and seemed to be a step in the right direction from an earlier spacing we used with a higher density. Still, I couldn't help but wonder if by keeping our spacing relatively tight, we might be missing out on growing some really huge garlic. Therefore, we designated 4 test plots for our garlic trial and set out to compare the results of using 4 different plant spacings. The different spacings we decided to test are outlined in the image below. You can see that we kept the number of rows consistent in each test bed, but varied the spacing within the rows from 3 to 9 inches. This variance allowed us to test plant densities from 2.16 bulbs/ft² to 6.32 bulbs/ft²
Each bed was 30 inches wide and 10 feet long for a total growing area of 25 square feet. Two drip lines supplied water to each bed and the drip lines were served by the same irrigation system. Beds were located in full sun so there were no inconsistencies do to shade from nearby buildings or trees. The dates were the same for the planting, scape removal, final harvest, and curing time.
If you are curious about the planting procedure, you can see me plant these beds in the video below.
Results and Discussion
All of the garlic was harvested on August 8 and hung in our nursery greenhouse to cure for several weeks. On September 1, all of the stems were cut an inch above the bulb. Then the total mass of bulbs from each test plot was weighed. The measurements from the 4 test plots are included below.
One slight modification to note is that the 5 inch bed ended up having an average spacing of 5.5 inches. I am not sure why. I just know that I harvested 86 bulbs from that bed and a spacing of 5 inches in the rows would have produced 96 heads instead. Perhaps one of my row markers was misplaced on my rake when I marked the bed. It's a mystery. Thankfully, a little math can still tell us that the actual average spacing there was 5.5 inches in the rows.
You can get a sense of the differences between the plots with the data shown above, but here's a visual to go along with that. Below, you can see how different the heads look between the 9 inch row spacing and the 3 inch row spacing. The bulbs from the 3 inch plot were tiny by comparison, but because there were so many of them, that plot actually produced the highest yield.
Have I mentioned that I love graphs? Here's where we get to see some real clarity from the numbers.
First, let's look at the red data representing the total mass of garlic bulbs harvested from each test plot. The line of best fit clearly indicates that the total mass increased as the planting density increased.
Next, let's turn our attention to the blue data which represents the average mass of the garlic bulbs at each planting density. Here we see the opposite trend. As the planting density increased, the size of the bulbs decreased.
At this point, it should be clear that if you want to maximize your total yield, you should plant your bulbs as tight as 3 inches in the row in rows 7.5 inches apart, but do so knowing that many of your bulbs are going to be embarrassingly small. There may even be increased total yields all the way up to the 9 bulbs/ft² proposed by John Jeavons. We really didn't see a peak in total yields in the spacings we chose to test.
It should also be clear now that if you are after the largest heads, use a more generous spacing. Our data showed an increase in bulb size all the way through to our widest spacing of 9 inches between heads in the rows and 7.5 inches between rows. Perhaps an even wider spacing would have produced a higher average bulb weight, but with this spacing we are already reducing the total yield significantly and not seeing much increase in bulb size, so I don't feel the need to push the spacing any wider. However, if you just want to produce large bulbs and you have a tonne of space to use so you don't care about your total yield per square foot, an extra wide spacing would be the way to go.
So where's the sweet spot in terms of spacing? If we look at both trends together, we can see that a planting density of 3.4 bulbs/ft² does seem to be an actual sweet spot. Both the red and blue data points at this planting density are slightly higher than the trend. Therefore, if we want to get the best of both worlds, our data would indicate that the best spacing is 3.5 bulbs/ft², which we achieved here with 4 rows 7.5 inches apart and 5.5 inch in row spacing.
These results are kind of comforting for me to see because I now know that I haven't been missing out on a more productive spacing over the last few years. Our current spacing of 6 inches in the row and 7.5 inches between rows works out to 3.2 bulbs/ft² and that is sure close to the sweet spot circled above. If anything, I am tempted to increase our planting density a little to achieve a higher total yield, but there are two considerations holding me back.
One is that I also know that bulb size is dependent on the size of the cloves planted the previous fall. If I committed all of my space to high density planting, I would achieve a high yield the first year, but most of my bulbs would be small, leaving me with only small cloves to plant for the following year's crop. Then the following year, my total yield would drop because I was forced to start with smaller cloves.
The second consideration is planting cost. The higher density beds cost a lot more to plant. The average mass of the cloves planted in this trial was 11.3 grams. That means the bed with 3 inch row spacing used 11.3 g/clove x 156 cloves = 1763 grams of seed to produce a total yield of 6.85 kg. That's only a 388% increase in mass from growing out the cloves, and it's also an expensive bed to plant considering that seed garlic is selling for around $50/kg in Canada these days if you're lucky. That means it would cost about $88 to plant that 25 square foot bed at 3 inch row spacing. The next bed in the trial with 5.5 inch row spacing used 11.3g/clove x 86 cloves = 971 grams of seed to produce a total yield of 6.10 kg. That's already a much better 628% increase in mass from our initial investment. With this slightly larger spacing it would only take $49 to plant the same 25 square foot area, and we would still end up with almost the same amount of garlic.
Those two considerations just bring me back to our sweet spot spacing of 3.4 bulbs/ft², which is almost the exact spacing we use today. A density close to this range will give me a relatively high total yield, a respectable average bulb size, and still provide me with a few large sized bulbs as outliers that I can set aside to seed the next year's crop. How's that for a lot of work for nothing? Well, I suppose it wasn't for nothing. It was for the assurance that we were making the best use of our land and getting the most from our garlic beds. Now go forth and spread the science-based truth to your garlic growing friends, and remember that the spacing is just one of the core variables impacting the growth of your garlic. I cover the other 4 core variables in part 3 of my free mini course. Members can find more specific instructions for planting and growing hardneck garlic in this Classroom post.