To Get a Quick General Idea:
This is a rule of thumb we find works well. Take the dimensions, and multiply the length in feet times the width, to get the square footage of your planting area.
Example: a 6’ by 10’ bed would equal 60 sq ft. Average plant spacing is about 18” so:
Divide square footage by 3. Example: 60 / 3 = 20 plants. This is the lower range for the number of plants.
Divide square footage by 2. Example: 60 / 2 = 30 plants. This is the upper range.
So if you add up the number of plants you are considering and it falls within the range then you are probably good. This is a down and dirty (get it? gardening humor) ballpark number to use.
Your bed is not a rectangle? You can sketch out the dimensions on a piece of graph paper (1 box per foot), then count the number of boxes within the outline of your garden bed. Use that number and divide as above.
Don’t Like Math or Measuring or Sketching on Graph Paper?
If your bed is ready, you can envision each average plant as a 5 gallon bucket. If the dirt is smooth, you can overturn a 5 gallon bucket and mark rings in the soil and count the rings. Got a shrub or large plant in your thinking? Put a lawn chair (or two) there – then make circles around it with the bucket. Medium size plant? Try a laundry basket. This can really help visualize your design as well (but I can’t guarantee the neighbors won’t wonder what you are up to!)
To Get a More Exact Answer:
Take the graph paper approach, but use your actual plant choices and their plant spacing to lay out circles on your graph paper. A plant with a 2’ spacing will need a 2’ diameter circle. If you actually cut out circles, you can move them around to figure out your preferred layout and save on erasers. See our Tips and Tricks section on Design for more info on design.