Lake Forest, IL planting calendar
- USDA zone
- 6a−10 to −5 °F
- Last frost
- Apr 26avg, 32°F
- First frost
- Oct 21avg, 32°F
- Growing season
- 176days
Lake Forest, Illinois is in USDA plant hardiness zone 6a. Its average last spring frost is around April 26 and the first fall frost around October 21, giving a growing season of about 176 days (NOAA 1991–2020 normals, 32°F, median). Start tender crops like tomatoes and peppers indoors weeks before the last frost and set them out afterward; sow hardy crops such as peas, spinach, and lettuce before it. The planner below turns those frost dates into a printable per-crop planting calendar.
Lake Forest planting calendar
Each crop's windows are counted from Lake Forest's average frost dates. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees.
- Start indoors
- Plant out
- Fall sowing
- First harvest
| Crop | Frost tolerance | Start indoors | Plant out | First harvest | Fall planting |
|---|---|---|---|---|---|
| Tomato | Tender | Mar 1 – Mar 15 | May 3 – May 10 | Jul 2 – Jul 22 | — |
| Pepper | Very tender | Feb 15 – Mar 1 | May 10 – May 17 | Jul 9 – Aug 8 | — |
| Cucumber | Tender | Mar 29 – Apr 5 | May 3 – May 10 | Jun 22 – Jul 12 | — |
| Summer squash / zucchini | Tender | — | May 3 – May 10 | Jun 17 – Jul 2 | — |
| Bush bean | Tender | — | May 3 – May 10 | Jun 22 – Jul 2 | Aug 22 – Sep 1 |
| Sweet corn | Tender | — | Apr 26 – May 10 | Jun 25 – Jul 25 | — |
| Basil | Very tender | Mar 15 – Mar 29 | May 3 – May 10 | Jun 2 – Jun 17 | — |
| Lettuce | Half-hardy | Mar 15 – Mar 29 | Mar 29 – Apr 12 | May 13 – May 28 | Aug 8 – Aug 23 |
| Pea | Hardy | — | Mar 15 – Mar 29 | May 9 – May 24 | Jul 29 – Aug 13 |
| Spinach | Hardy | — | Mar 15 – Mar 29 | Apr 24 – May 4 | Aug 18 – Aug 28 |
| Carrot | Half-hardy | — | Apr 5 – Apr 12 | Jun 4 – Jun 24 | Jul 19 – Aug 8 |
| Broccoli | Half-hardy | Mar 1 – Mar 15 | Mar 29 – Apr 12 | May 23 – Jun 12 | Jul 24 – Aug 13 |
Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.
Frost & freeze dates
From NOAA's 1991–2020 Climate Normals at station USC00111497. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 (p10 for spring, p90 for fall) — the conservative date to plant after or harvest before.
| Threshold | Last spring — avg | Last spring — 90%-safe | First fall — avg | First fall — 90%-safe | Season (days) |
|---|---|---|---|---|---|
| 36°F | May 8 | May 26 | Oct 10 | Oct 24 | 154 |
| 32°F (freeze) | Apr 26 | May 11 | Oct 21 | Nov 2 | 176 |
| 28°F | Apr 13 | Apr 28 | Nov 1 | Nov 14 | 202 |
| 24°F | Mar 31 | Apr 16 | Nov 12 | Nov 28 | 225 |
32°F is the standard "freeze" line that damages tender crops; lighter 36°F frost can nip the most cold-sensitive plants, while hardy crops shrug off light frost down toward 28°F. Use the threshold that matches what you are protecting.
Growing degree days
Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. Warm-season crops need a long, warm GDD total; a short, cool GDD total favors greens and brassicas.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 2,963 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 5,084 | cool-season crops (brassicas, greens) |
Hardiness zone 6a
Lake Forest sits in USDA plant hardiness zone 6a on the 2023 map — meaning its average annual extreme minimum winter temperature is about −10 to −5 °F. That number tells you which perennials, shrubs, and trees reliably survive an average winter here; it does not set your planting dates, which come from the frost calendar above.
Explore more places in zone 6a, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Lake Forest?
- Lake Forest, Illinois is in USDA plant hardiness zone 6a on the 2023 map (average annual extreme minimum temperature −10 to −5 °F) — from the USDA Plant Hardiness Zone Map 2023, matched to this location's ZIP. See the methodology page for sources.
- When is the last frost in Lake Forest?
- The average (median) last spring frost at 32°F is around April 26, from NOAA's 1991–2020 climate normals at the nearest reporting station. Roughly one year in ten the last frost is as late as May 11, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Lake Forest?
- The average first fall frost at 32°F is around October 21. That leaves a growing season of about 176 days between the average last spring and first fall frosts.
- When should I start tomatoes in Lake Forest?
- Start tomato seeds indoors about Mar 1 – Mar 15 and transplant them outside about May 3 – May 10, once the danger of frost has passed. Estimated first harvest is around Jul 2 – Jul 22.
- How long is the growing season in Lake Forest?
- About 176 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~April 26) and the average first fall frost (~October 21). Cold-hardy crops extend usable time at both ends; frost-tender crops fit inside it.
Sources & method
Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020 for station USC00111497 (Chicago Botanic Garden, 2.1 km away). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to this location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.