Part of why I created this blog was to record the answers to gardening questions I’ve had along the way. As a person who loves research, I thought it would be handy to have that research stored together, in a place where I could also share it with others. I encourage you to do your own research on anything I have posted, and I’ve included footnotes with my sources for information that isn’t based on personal experience—both so I can go back and reference the original source for something, and so you can do the same. Let me know if you spot any errors, so I can correct them!
This week, I’ve been thinking a lot about how many seeds I should be planting when I’m starting seeds indoors. I planted some Cortland Onions over the weekend, but I’m not sure if I planted the right number of seeds. At the time, I decided to just guess, because a) I was excited to get some seeds in soil, and b) we love onions, so if I plant too many, I can always adjust my garden plans to fit them in somewhere.
For this year’s garden, I’m planning on two 4 x 8 raised beds, plus a few pots. However, the spacing instructions on all of my seed packets assumes that I will be planting in rows, and that I need to be able to walk between those rows in order to access the plants. With raised beds that are only four feet wide, I’ll easily be able to reach the centre of the bed—no need for rows, nor the space between them. I can plant my seeds with just enough room to grow, based on area rather than row spacing. But without being able to rely solely on the instructions on my seed packets, how do I know how many I should plant?
In this blog post, I’m going to discuss how to solve this problem and answer the following questions:
- How can the Square Foot Gardening method inform how I plant my seeds?
- What is the purpose of the row and seed spacing guidelines on a seed packet?
- How do I calculate the amount of space that each plant requires?
- How many seeds should I plant in a square-foot section of planting space?
If you’d rather skip straight to the answer, without all of the explanations in between, I’ve created an easy-to-reference Square Foot Gardening Plant Spacing Cheat Sheet at the end of this post—click here to jump straight to that section!
How can the Square Foot Gardening method inform how I plant my seeds?
If you’ve heard of Square Foot Gardening, you’ll know that area-based planting is at the core of the philosophy. However, I’m not interested in following the program exactly, nor without some understanding of the underlying rationale—I want to know the WHY as well as the HOW.
Digging into the information available through the Square Foot Gardening website and book, however, it was much harder to find an explanation than I expected. They have planting charts readily available, but they don’t explain where these recommendations came from. For onions, the chart simply recommends planting 16 plants per square foot; in the book, they recommend 9 to 16 per square foot, depending on the variety you’ve chosen.1
The closest thing to an explanation I could find was this video about the space savings provided by the Square Foot Gardening method. In the demonstration, they appear to calculate the area required for each plant using the seed spacing guidelines on the seed packet.2
What is the purpose of the row and seed spacing guidelines on a seed packet?
The seed companies that I’ve looked at—West Coast Seeds, High Mowing Organic Seeds, and McKenzie Seeds—don’t seem to share the details about how the information on their seed packets is decided on, nor how to convert it to an area-based method. The latter seemed especially surprising, as I was under the impression that Square Foot Gardening was gaining significant steam, but perhaps that’s just the areas of the internet I hang out in. Regardless, all of their advice seems to assume you’ll be planting your seeds in rows, so this was a bit of a dead end.
How many seeds should I plant in a square-foot section of planting space?
In the end, I found this article from Garden in Minutes, which seems to back up my assumptions from the Square Foot Gardening video, as well as the information I found on several other gardening blogs (with varying degrees of specificity).
According to Garden in Minutes, the math would go something like the following 4 (If formulas freak you out, keep scrolling down to find the Square Foot Gardening Plant Spacing Cheat Sheet that I created as a quick reference for planting!):
(width of planting section) / (packet seed spacing)
x
(length of planting section) / (packet seed spacing)
For a packet with seed spacing of three inches, and a square foot section of your garden, the calculation would look something like this:
(12 inches / 3 inches) x (12 inches) x (3 inches)
= 4 x 4
= 16 plants
Simple enough, right? However, there remains one flaw with this formula that we still need to address: the amount of space required by each individual plant, including the space around the edges. We can see this easily if we draw out the grid we would be planting within, like in the following illustration:
If you are planting 16 seeds in a square foot, you would plant them in a grid with four seeds across and four seeds down. However, the last row of seeds on the right and on the bottom would end up right on the edge of your square foot if you’re keeping three inches between each seed—not ideal if you have an irrigation grid like the one sold by Garden in Minutes (yes, I want one; no, I do not need one lol), and not ideal if the next section over is tightly planted in a similar fashion.
How do I calculate the amount of space that each plant requires?
We need to have three inches of space around all four sides of our seed, which means our formula needs to take that into account. I’ve added it to the formula as follows:
[(width of planting section – packet seed spacing) / (packet seed spacing)]
x
[(length of planting section – packet seed spacing) / (packet seed spacing)]
For a packet with seed spacing of three inches, and a square foot section of your garden, the calculation would look something like this:
[(12 inches – 3 inches) / (3 inches)] x [(12 inches – 3 inches) / (3 inches)]
= (9 / 3) x (9 / 3)
= 3 x 3
= 9 plants
And if we draw this one out, with 9 seeds, we can see that we now have three inches of space around all four sides of the plant:
These calculations could also obviously be used for garden sections in any size you’d like, not just for a section that is a square foot.
Now, obviously, this is a lot of brain-bending math, and you probably don’t want to take your calculator with you to the garden—I know I don’t. To make planting easier, I created the cheat sheet below using the seed spacing recommendations I found on the seeds I’m planning to plant myself this year.
Square Foot Gardening Plant Spacing Cheat Sheet
Note that because I’m in Canada, and many of the seed packets I have use centimetres instead of inches, I’ve included that measurement first on the cheat sheet below. Some of the seed spacings for the varieties listed below depend on how large you want the vegetable to be (e.g., carrots, beets, onions), so make sure to read your own seed packets to determine what’s best for your garden!
| Seed Spacing | Size of Section | # of Plants Per Section | Grid to Plant | Example Seed Varieties |
| Copied from packet. | Length x width of the section you’ll be planting in. | The total number to plant in this section. | The number of seeds or seedlings to plant across and down. | These are some of the varieties I’m planning to plant. |
| 2.5 cm (1 in) | 30 cm by 30 cm (12 in by 12 in) | 121 plants | 11 by 11 | Starburst Carrot Blend (High Mowing) |
| 5 cm (2 in) | 30 cm by 30 cm (12 in by 12 in) | 25 plants | 5 by 5 | Purple Plum Radish (Sage Garden) |
| 7.5 cm (3 in) | 30 cm by 30 cm (12 in by 12 in) | 9 plants | 3 by 3 | Touchstone Gold Beet (High Mowing) |
| 10 cm (4 in) | 30 cm by 30 cm (12 in by 12 in) | 4 plants | 2 by 2 | Greensleeves Dill (High Mowing) |
| 12.5 cm (5 in) | 30 cm by 30 cm (12 in by 12 in) | 1 plant | Plant one in the centre of the section. | Cortland Onions (High Mowing) |
| 15 cm (6 in) | 30 cm by 30 cm (12 in by 12 in) | 1 plant | Plant one in the centre of the section. | Giant of Italy Italian Parsley (The Incredible Seed) |
| *Note increased section size (equivalent of four square-foot sections). | ||||
| 17.5 cm (7 in) | 60 cm by 60 cm (24 in by 24 in) | 4 plants | 2 by 2 | None from this year’s list! |
| 20 cm (8 in) | 60 cm by 60 cm (24 in by 24 in) | 4 plants | 2 by 2 | Buttercrunch Lettuce (McKenzie Seeds) |
| 23 cm (9 in) | 60 cm by 60 cm (24 in by 24 in) | 1 plant | Plant one in the centre of the section (extra space for smaller plants around edges). | Phacelia Bee’s Friend (Fruition Seeds) |
| 25.5 cm (10 in) | 60 cm by 60 cm (24 in by 24 in) | 1 plant | Plant one in the centre of the section (extra space for smaller plants around edges). | Two Star Lettuce (High Mowing) |
| 28 cm (11 in) | 60 cm by 60 cm (24 in by 24 in) | 1 plant | Plant one in the centre of the section (extra space for smaller plants around edges). | Red Salad Bowl Lettuce (High Mowing) |
| 30 cm (12 in) | 60 cm by 60 cm (24 in by 24 in) | 1 plant | Plant one in the centre of the section (extra space for smaller plants around edges). | Mammoth Sunflower (Burpee) |
Download a printable PDF of this cheat sheet here to take with you into the garden (please forgive my design skills on this one—they’re very much a work in progress!).
Did you find this tool helpful? Would you like more spacing options added to the chart? Let me know in the comments!
Now if you’ll excuse me, I need to go plant some more onions.
Sources
1 Bartholomew, Mel, & the Square Foot Gardening Foundation. (2018). All new sqaure foot gardening (3rd edition, pp. 240). Cool Springs Press.
2 Square Foot Gardening Foundation)—OFFICIAL. (2022, January 7). SFG Essentials | Grow More in 80% Less Space [Video]. YouTube. https://www.youtube.com/watch?v=B-I0-Bkas_M
3 Judd, Angela. (n.d.). Plant Spacing in Square Foot Gardens. Growing in the Garden. Retrieved March 6, 2023 from https://growinginthegarden.com/plant-spacing-in-square-foot-gardens/
4 Garden in Minutes. (2023, February 10). The All-In-One, Square Foot Gardening Plant Spacing Guide w/ Printable Chart (60+ Plants!). Retrieved March 6, 2023 from https://gardeninminutes.com/plant-spacing-chart-square-foot-gardening/
Playing in the Dirt 