Groundcover Spacing Calculator: Plants Needed

This calculator works out how many groundcover plants you need for a given area, and the spacing between them, based on the mature width of the plant you choose. Enter your planting area’s length and width, the mature spread of the plant, and the calculator returns the number of plants required and the on-center spacing. It uses the standard square-grid spacing formula, which is the most common method for mass plantings.

Formula

The calculator uses the square-grid spacing formula, which places plants in rows and columns with equal spacing in both directions. The formula is:

Plants Needed = ⌈(Area Length × 12 / Plant Spread) × (Area Width × 12 / Plant Spread)⌉

Where:

• Area Length – the length of the planting bed in feet (ft)
• Area Width – the width of the planting bed in feet (ft)
• Plant Spread – the mature width of a single plant in inches (in)
• ⌈ ⌉ – the ceiling function, which rounds up to the nearest whole number, because you can’t buy a fraction of a plant

The result is the number of plants needed for a full, dense groundcover when planted on-center (the distance from the center of one plant to the center of the next) equal to the mature spread.

Worked Example

Using the default values: area length = 20 ft, area width = 10 ft, plant spread = 12 in.

1. Convert the length to inches: 20 ft × 12 = 240 in
2. Convert the width to inches: 10 ft × 12 = 120 in
3. Divide length by spread: 240 / 12 = 20 plants along the length
4. Divide width by spread: 120 / 12 = 10 plants along the width
5. Multiply: 20 × 10 = 200 plants

So the calculator returns 200 plants for a 20 ft × 10 ft area with 12-inch spacing. The on-center spacing is simply the mature spread (12 in).

How to Choose the Right Spacing

The most common mistake is spacing plants too far apart, which leaves bare soil and invites weeds. The general rule for groundcovers is to space them at a distance equal to their mature spread. For example, if a plant grows 18 inches wide, space plants 18 inches apart on-center. This creates a solid mat within one to two growing seasons.

If you want faster coverage, you can space them closer (e.g., 75% of mature spread), but that increases cost. If you’re patient or on a budget, you can space them up to 1.5 times the mature spread, but you’ll wait longer and may need mulch to suppress weeds in the gaps.

Common Mistakes and How to Avoid Them

• Unit mix-ups: The calculator expects area in feet and spread in inches. If you measure the area in yards or meters, convert first (1 yd = 3 ft, 1 m ≈ 3.281 ft).
• Forgetting the ceiling: Always round up, not down. If you need 199.2 plants, you must buy 200.
• Using the pot size instead of mature spread: The spread in the calculator is the mature width of the plant, not the size of the nursery pot. Check the plant tag or a reliable source.
• Ignoring irregular shapes: The calculator assumes a rectangular bed. For curved or L-shaped areas, break them into rectangles and add the results, or use a more generous estimate.

Square vs. Triangular Spacing

This calculator uses square spacing (grid pattern), which is the standard for most groundcovers. However, some landscapers use triangular (staggered) spacing, which places plants in a hexagonal pattern. Triangular spacing requires about 15% more plants for the same area and spacing, but it can create a more uniform cover and reduce erosion on slopes. If you prefer triangular spacing, multiply the result by 1.15.

Cost and Budget Considerations

Once you know the number of plants, you can estimate cost by multiplying by the price per plant at your local nursery. Prices vary widely by species, pot size, and region. For example, if you need 200 plants and each costs $4, the total is $800. Don’t forget to add 5-10% for replacements in case some plants don’t survive. Also consider the cost of mulch, compost, and irrigation if needed.

Alternatives to Traditional Groundcovers

If you’re looking for low-maintenance options, consider these alternatives:

• Mulch: Bark, wood chips, or gravel can cover bare soil without any plants. It’s cheaper upfront but needs replenishing every few years.
• Grass lawns: A traditional lawn is a groundcover, but it requires mowing, watering, and fertilizing.
• Native groundcovers: Many native plants are drought-tolerant and support local wildlife, but they may be harder to find in nurseries.
• Hardscaping: Patios, paths, or decorative rock eliminate the need for plants entirely, but they don’t provide the same ecological benefits.

Frequently Asked Questions

How do I measure the mature spread of a plant?

Check the plant tag or label that comes with the plant. It usually lists the mature height and spread. If not, look up the species in a reliable gardening book or website. For example, a common groundcover like creeping thyme typically spreads 6-12 inches.

Can I use this calculator for any type of groundcover?

Yes, the formula works for any plant as long as you know its mature spread. It’s commonly used for perennials, shrubs, and ornamental grasses. For very large shrubs, you might want to use a larger spacing to avoid overcrowding.

What if my planting area is not a perfect rectangle?

Break the area into smaller rectangles, calculate each separately, and add the results. For irregular shapes, you can also use the average length and width, but this may over- or under-estimate. When in doubt, round up to the nearest whole plant.

How long does it take for groundcover to fill in?

It depends on the plant species, soil, water, and sun. Most groundcovers fill in within 1-2 growing seasons when spaced at their mature spread. Faster-growing types like creeping Jenny may fill in within a year, while slower ones like some ferns may take 3 years.

Should I space plants closer on slopes?

Yes, on slopes, closer spacing (e.g., 75% of mature spread) helps stabilize the soil faster and prevents erosion. You can also use erosion-control blankets or netting. The calculator can be used with a smaller spread value to simulate closer spacing.

What does “on-center” spacing mean?

On-center spacing measures the distance from the center of one plant to the center of the next. This is the standard way to space plants because it’s easier to measure than edge-to-edge. For example, if you plant at 12 inches on-center, the centers are 12 inches apart, leaving a small gap between the edges of young plants.

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in length | Area Length | ft | 20 | 0
in width | Area Width | ft | 10 | 0
in spread | Mature Plant Spread | in | 12 | 1
out plants | Plants Needed | plants | ceil((length*12/spread) * (width*12/spread)) | 0
out spacing | On-Center Spacing | in | spread | 0