There are different ways to calculate the amount of plants you’ll need. I usually use the mature width or spacing recommendation from the plant tag and square footage of the planting bed. When planting groupings of the same plant you can just use that number. When calculating the spacing for different plants add half of each plant’s mature width. For example: if one plant’s mature width is 2’ and the other is 4’ you would space them 3’ apart (2’+4’ divided by 2 = 3’). Just remember to dig the holes a little less far apart to have some attractive overlap.
Next, it’s time to measure your planting beds. The easiest way would be to measure each planting bed separately. If you have different plant spacing within a bed (ex: one group of plants requires 2’spacing and another group 3’ spacing), then measure each of those sub-areas separately. Then when you have those measurements just calculate the area of each bed or each bed sub-area (length x width). Then just plug those numbers into a plant spacing calculator.
You can find many plant spacing calculators online by doing a search, but here are few that I found that look good:
I tested the three plant calculators and got the same result for the first two. The third one gave me drastically lower number of plants, but they use a staggered planting method. The pros of this site are that you can account for trees in your beds and they have separate mulch, annual, stone and mowing calculators. Also if you want to use the traditional row planting method you can get those results from them too.
Savings tip: By using the staggered planting method you will need less plants and your garden will have a more natural look. I generally use this method when I’m short on space and don’t want to spend a lot on plants.
You can also consider splitting the plants in order to save even more.