Mass Those Perennials Right with a Handy Chart

Do you hate math but ever wonder how many plants you would need to fill up an area if they were placed at recommended spacing?

And want to know how to calculate how many annuals, shrubs or perennials you would need to cover a certain area if you mass them out?  It is generally only something for large residential or commercial properties, but if you are planting a feature bed and you know the spacing required (from the plant tags or catalog), it actually is quite easy to figure out. Be a pro too.

In the photo above of the large residential project, there are Buffalo Juniper, ‘Apple Blossom’ carpet roses, ‘Iceberg’ Floribunda roses,  Vinca minor and Impatiens that are newly planted and massed in large areas. The spirea and the ‘Annabelle’ hydrangea are planted in alternating rows backed with arborvitae.  So you can see we have ground covers, annuals and shrubs that need accurate spacing to fill in as a mat of plants. It would be rare to do this sized planting at a home, but the project was designed for an outdoor wedding and reception. But common to home planting is ground covers.

First determine the square footage you want to cover. Let’s assume you have a 10 foot by 10 foot bed to make it simple. So we have 1oo square feet.  Now say we want to place a ground cover 8 inches on center from the neighboring plant, so our spacing becomes 8 inches.

We then use a spacing multiplier of 2.25 plants per square foot times our 100 square feet for The Number of Plants Needed with the equation of:

100 sq. ft. x 2.25 = 225 plants. If a flat of annuals is in sets of 48, you need 225 divided by 48 for 4.69 flats. Most ground covers come in flats of 50 or 100 so you can see we need 4.5 sets of 50. Use the Handy Chart I made and download the pfd with the link.


To find an unknown multiplier, say 13 inches, use this equation:

You can see that 13 can be a mathematical ‘x’ number and easily replaced with say 14 inches of spacing. So at 13 inches on center, we need of 85 plants for our easy to calculate 10 ft. x 10 ft. area, and using the formula above for The Number of Plants Needed. The more common spacings are on the chart, but I left out 16 inches so you can try it. If you want to be economical, space them at 14 inches and you can use one flat of plugs of 72 plants. See how design works and how this makes buying decisions simple?

But what if we have odd-shaped beds…. break them down into shapes you can calculate to the closest dimensions, like circles, rectangles and triangles, and add up the known areas (shown in red-dashed lines on the diagram). What you will notice in the diagram is that the geometric shapes overlap. This overlap makes up for the areas not covered by the circles, triangles and a square. You get used to this approximation and get very accurate with it over time.

Or use a CAD program to design like I do and it tells me exactly how big a bed I have to a couple of decimals.  My bean bed has a perimeter of 75.9 feet and an area of 297.7 square feet at 1/4 inch scale. Nifty huh? So how may Pachysandra might I need to place under two small trees anchoring both ends of the imaginary bean-shaped bed? You can design in some pretty perennials to this bed for me. But then, minus them so as to reduce the Pachysandra necessary.

Well, just remember to leave an area free of plants around each tree too, say a 3 foot diameter circle free of plants, shown in blue-dashed lines. Easy to figure this out with some geometry.

So that means we minus 14 square feet (the two blue-dashed lines) for two trees from the total. Remember the equation to the left?

Sorry about the odd pi sign, but trying to do this on a post gave a good π, but making a superscript number in a post on a Mac with standard keystrokes makes the post editing window grow and grow. Took me a little time to figure out how to shrink it back down too after I grew it right off my big 30 inch monitor. Talk about mild panic. What I though would fix it didn’t until I finally got the right keystrokes. Oh what we learn by mistake….

Just a Tip to pass along….. so your quantity plant buying is easier.

And to all you Garden Walk Buffalo followers, there is more to come. See the Niagara Falls Garden magazine coming soon.


About Garden Walk Garden Talk

I love to photograph, paint, draw, design, garden, travel the world, and pass on a few tips and ideas that I learned through experience as a Master Gardener and architect. I am highly trained in my field and enjoy my work each and every day. I garden in Niagara Falls, NY in zone 6-B. Find me at:
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15 Responses to Mass Those Perennials Right with a Handy Chart

  1. Math makes everything better.

  2. Chad B says:

    I feel dumb now. But smarter too . . . Thanks?

  3. skeeter says:

    Gee, it looks so confusing and I was good at math in… I do at times use math in the garden when planting though..

  4. Donna says:

    alright I have a headache…not too bad…I have always struggled to get this right so I appreciate the math and I love math…my husband will love this too and I may make him in charge of figuring this out lol 🙂

  5. OK, that is getting bookmarked for future reference!

  6. Greggo says:

    it reminds me of being back in school..

  7. Wow, that is very helpful. I’m a lot more casual with my plant planning, but there are lots of pointers in there that perhaps I can figure out and use.

  8. Masha says:

    I hope one day I will have a garden where I can use this kind of math. That bed looks beatiful.

  9. One says:

    That is a gorgeous bed.

    In my garden, I grow one Cosmos, one Zinnia, one Basil… Now, they have become uncountable. All spaces are filled up.

  10. Andrea says:

    Haha, i only tried to learn about that during student days, i can even recall quincunx method. But now i am not graded anymore so i wont try thinking about that again. I would like to plant only a 1X1m spacing on a hectare, which is 10,000 sq meters. I haven’t also done a superscript or subscript as in C2H4 in blogging, as i might not be able to do that. LOL. But i am sure some people will find this informative and useful.

  11. Holley says:

    Thanks for the tip! I may try this formula on the next bed I put in – if it ever gets cool enough to work outside again!

  12. b-a-g says:

    Donna – I would need a revised Handy Chart which factors in slug damage.

  13. Karen says:

    Great way to do more than guesstimate when a bed needs planting. I am going to bookmark this post for future reference, Donna. Wonderful job on the pi symbol, too!

  14. Bridget says:

    Ahhh! I still hate maths.

  15. Geometry is a wonderful thing. Hooray for Euclid!
    The geometrical principle I use the most in the garden is actually the Pythagorean Triple which makes it so very simple to determine right angles with a piece of string and three pegs.

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