Answer: Most vegetable and ornamental plants have an average mature size you can use to figure this out. If say, the mature width is two feet and you want a "full" planting, then you would plant them two feet apart. If you measure the area, you can get a general idea for how many plants you need.
If you want bedding plants or perennials to look full faster, plant them a bit closer together. If you like your plants spaced so you can see soil between them, then space them a bit further apart.
If the plant is a spreading groundcover such as English ivy, the closer you plant the little starter plants the faster they will fill in and cover the area. For quick cover some folks will put them as close together as one every foot. To save money some folks will space them two feet apart.
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