Milkweed Planting Calculator
Milkweed plugs, flats, and seed packets for monarch beds.
What it does
Monarchs raise families only on milkweed. A counted patch of the right plants feeds more caterpillars than scattered singles.
The tool above divides bed area by an 18 in grid. One hundred square feet plants 44 plugs from 1 flat, or 2 seed packets direct sown. The metric match is 9.3 m2 with the same counts.
Spacing suits hungry mouths. Eighteen inch drifts shade weeds and hold moisture, while wider spacing starves caterpillars between plants.
Plugs beat packets for instant patches. Fifty cell flats transplant after frost, while seed needs cold stratification and a patient spring.
Clean plants matter most. Systemic pesticides ride nursery stock into caterpillars, so ask before buying.
How to use
- Pick imperial (sq ft) or metric (m2).
- Enter bed area: 100 sq ft suits a starter patch.
- Read plants first.
- Read flats next for plug orders.
- Read packets for direct sow.
- Plant pesticide free and water year one.
Formula
- area
- Bed area.
Metric uses 0.21 m2 per plant on a 45 cm grid with matching flat and packet counts.
Worked example
Example: 100 sq ft bed
Imperial defaults: 100 sq ft bed. These are also the default values, so the page opens on this answer.
- Bed: 100 sq ft.
- Grid: 18 in, so 2.25 sq ft per plant.
- Plants: round(100 / 2.25) = 44 plants.
- Flats: ceil(44 / 50) = 1 flat.
- Packets: ceil(100 / 50) = 2 packets.
- Tool shows 44 plants, 1 flat (50 cells), 2 seed packets.
Reference table
| Bed | Plants | Flats |
|---|---|---|
| 50 sq ft | 22 plants | 1 flat (50 cells) |
| 100 sq ft | 44 plants | 1 flat (50 cells) |
| 200 sq ft | 89 plants | 2 flats (50 cells) |
| 9.3 m2 | 44 plants | 1 flat (50 cells) |
Limits
- Count math only. Species must suit the region.
- First year sleep, then leap.
- Pesticides void the plan.
- Deer browse needs fencing.
Common mistakes
- Tropical milkweed that skips migration cues.
- Sprayed nursery stock.
- One plant per lonely corner.
- No water in year one.
- Mowing fall stems with eggs aboard.
Try it yourself
Quick checks. Answers sit tucked under each question.
200 sq ft?
round(200 / 2.25) = 89 plants (88.9 rounds up). Flats are ceil(89 / 50) = 2 flats.
20 m2 metric?
round(20 / 0.21) = 95 plants (95.2 rounds down). Packets are ceil(20 / 5) = 4 packets.
Questions
Plants for 100 sq ft?
Divide area by 2.25 sq ft per plant. Here round(100 / 2.25) = 44 plants.
Nursery flats?
Divide plants by 50 cells. Here ceil(44 / 50) = 1 flat.
Seed packets?
One packet per 50 sq ft. Here ceil(100 / 50) = 2 packets.
Spacing?
Near 18 in spacing suits common milkweed plugs. Tight drifts feed more caterpillars per yard.
Stratify seeds?
Yes. Cold stratify seed 30 days moist in the fridge before spring sowing.
Any milkweed works?
No. Pesticide free plants only; nursery stock needs checking.