How many bulbs per square foot, and how many packs for your bed

How the bulb count works

Bulbs per square foot are 144 divided by the spacing in inches squared: at 5 in, 144 ÷ (5 × 5) = 5.76 bulbs per sq ft. Each bulb gets a 5 × 5 in square, and the bed area divided by that square gives the count. Bulb growers work the same way: Colorblends has you multiply the bed's square footage by the recommended bulbs per square foot.

On a staggered grid (triangular spacing), every other row shifts by half a spacing, each plant has six neighbors at the same distance and the rows move closer, to 0.866 times the spacing. Each plant then gets 5 × 4.33 in, and at the same spacing you plant 15.5% more: 6.65 per sq ft instead of 5.76.

The area is length × width, π × diameter² ÷ 4 for a round bed, or you type it in. The count is rounded up first, then the packs: with packs of 25 the calculator counts whole packs and the spare bulbs.

Worked example: an 8 × 4 ft bed of tulips

These are the values the calculator opens with in US units: an 8 × 4 ft bed, tulips 5 in apart on a square grid, packs of 25.

• Area: 8 × 4 = 32 sq ft
• Bulbs per sq ft: 144 ÷ (5 × 5) = 5.76
• Bulbs: 32 × 5.76 = 184.3, rounded up to 185 bulbs
• Packs: 185 ÷ 25 = 7.4, rounded up to 8 packs of 25, with 15 spare

On a staggered grid at the same spacing you need 213 bulbs, 9 packs of 25.

In the UK, in metres: a 2 × 1 m bed of tulips 12 cm apart has 1 ÷ (0.12 × 0.12) = 69.44 bulbs per m², so 2 × 69.44 = 138.9, rounded up to 139 bulbs, 6 packs of 25 with 11 spare. A round bed 1.50 m across is 1.77 m²: daffodils 15 cm apart need 79 bulbs.

Bulbs per square foot by type

The calculator's starting spacing, on a square and a staggered grid. For the bulbs, these fall inside the per square foot ranges on Colorblends' product pages: tulips 5 to 7, daffodils 4 to 5, crocuses 18 to 22, hyacinths and 'Purple Sensation' alliums 5 to 6, grape hyacinths 14 to 16.

Plant	Spacing	Square grid	Staggered
Tulips	5 in	5.76 per sq ft	6.65 per sq ft
Daffodils	6 in	4 per sq ft	4.62 per sq ft
Crocuses	2 3/4 in	19.04 per sq ft	21.99 per sq ft
Hyacinths, alliums	5 in	5.76 per sq ft	6.65 per sq ft
Grape hyacinths	3 in	16 per sq ft	18.48 per sq ft
Annuals	10 in	1.44 per sq ft	1.66 per sq ft
Ground cover	12 in	1 per sq ft	1.15 per sq ft
Perennials	18 in	0.44 per sq ft	0.51 per sq ft

For annuals, Mississippi State University gives 8 to 12 in for begonia and vinca, 6 to 12 in for marigold and 12 to 15 in for petunia. For perennials the RHS says to plant a little closer than the spread on the label: about 50 cm (18 in) for a plant 60 cm (2 ft) wide. Virginia Cooperative Extension puts most ground covers 6 in to 2 ft apart, the slow spreaders closer. Giant alliums such as 'Globemaster' go much wider: 10 in apart, 1 to 2 bulbs per sq ft.

In metric, the same spacings give 69 tulips per m² at 12 cm, 44 daffodils at 15 cm, 204 crocuses at 7 cm and 156 grape hyacinths at 8 cm.

Planting depth and packs

The RHS rule works for most bulbs: plant them at two to three times their own depth, so a bulb 2 in (5 cm) tall goes in a hole 4 to 6 in (10 to 15 cm) deep, nose up, with at least twice the bulb's width between bulbs. Colorblends gives the hole depth, measured from the soil surface to the bottom of the hole: 5 in for tulips, hyacinths and 'Purple Sensation' alliums, 6 in for daffodils, 3 in for crocuses and grape hyacinths. In heavy clay soils Colorado State University suggests planting 1 to 2 in higher.

Pack sizes differ from store to store, so type in the count on the pack you find. The calculator starts at 25 bulbs for tulips and daffodils, 50 for crocuses and grape hyacinths, 10 for hyacinths and alliums, 6-packs for annuals and single pots for perennials and ground cover. The receipt counts whole packs and the spare pieces. For a border that people will see, the RHS suggests groups of at least six bulbs and 25 to 50 for a good show.

Common mistakes

• Halving the spacing to double the bulbs. The count goes with the square: from 5 in to 2 1/2 in you go from 5.76 to 23.04 per sq ft, four times as many.
• Forgetting the staggered grid. With staggered rows at the same spacing you need 15.5% more: 213 tulips instead of 185 on 32 sq ft.
• Trusting the area count in a small or round bed. Part of the cells along the edge fall outside the bed: the drawing shows how many plants really fit, and in a small bed that can be a few less.
• Planting too shallow. Two to three times the bulb's height, measured to the bottom of the hole; a little less in clay.
• Naturalizing at bed spacing. For daffodils left to spread in grass, Colorblends says to space them wider than the bag label, so the bulbs have room to multiply.