Ornamental allium · Berwyn, Illinois, United States
When to plant ornamental alliums in Berwyn
Main outdoor window
6-20 Oct
A climate estimate. Check your soil, forecast and cultivar.
Timed from last spring frost
4 April
In Berwyn, on your chosen risk.
First flowers
1 May - 30 Jun
If you plant at the start of the window.
The average last frost. Half of all years are later than this.
Step by step in Berwyn
Download these planting dates (.ics)
Import into Apple Calendar, Google Calendar or Outlook. Dates use the selected risk level and are planning estimates, not frost alerts.
Each window is a planning range. Check the forecast, soil and cultivar before planting; a later date is not automatically safer in hot or wet conditions.
Plant the bulbs
6-20 Oct
Four to six weeks before your first frost, so roots form before the ground cools.
In flower
1 May - 30 Jun
Ornamental alliums through the year
Against the frost risk in Berwyn
Seasonal context
Rainfall and the growing season in Berwyn
May has the highest average precipitation (119 mm); January has the lowest (47 mm). Compare this pattern with the planting windows and your water supply.
During wetter months, check that beds and containers drain after heavy rain. During drier months, plan how you will keep seeds and new transplants watered. Monthly averages can hide long dry spells and intense storms; they do not show soil moisture, humidity or this week's weather.
Source: NASA POWER, corrected MERRA-2 precipitation, 2001–2020. Values are approximate monthly totals, including the water equivalent of snow, from the nearest 0.5° × 0.625° grid cell. This reference period differs from the temperature record. Rainfall provides context and does not change the calculated planting dates. Method and limitations.
Monthly growing conditions in Berwyn
Compare the planting window with the local temperature pattern. These are historical monthly averages, not this year's forecast or measured soil temperatures. Average frost days do not tell you whether a particular night will freeze.
| Month | Average low | Average high | Frost days |
|---|---|---|---|
| January | -4.9°C (23.2°F)23.2°F (-4.9°C) | -0.2°C (31.7°F)31.7°F (-0.2°C) | 25.7 |
| February | -4.5°C (24°F)24°F (-4.5°C) | 0.8°C (33.5°F)33.5°F (0.8°C) | 23.6 |
| March | -0.5°C (31°F)31°F (-0.5°C) | 5.1°C (41.2°F)41.2°F (5.1°C) | 16.9 |
| April | 3.8°C (38.8°F)38.8°F (3.8°C) | 10°C (50°F)50°F (10°C) | 2.7 |
| May | 8.8°C (47.8°F)47.8°F (8.8°C) | 15.2°C (59.3°F)59.3°F (15.2°C) | 0.0 |
| June | 15.1°C (59.1°F)59.1°F (15.1°C) | 21.1°C (70°F)70°F (21.1°C) | 0.0 |
| July | 19.6°C (67.3°F)67.3°F (19.6°C) | 24.9°C (76.8°F)76.8°F (24.9°C) | 0.0 |
| August | 20°C (68.1°F)68.1°F (20°C) | 25.1°C (77.1°F)77.1°F (25.1°C) | 0.0 |
| September | 16.4°C (61.5°F)61.5°F (16.4°C) | 22°C (71.6°F)71.6°F (22°C) | 0.0 |
| October | 9.9°C (49.7°F)49.7°F (9.9°C) | 15.6°C (60°F)60°F (15.6°C) | 0.0 |
| November | 3.4°C (38.2°F)38.2°F (3.4°C) | 8.5°C (47.3°F)47.3°F (8.5°C) | 5.5 |
| December | -1.8°C (28.9°F)28.9°F (-1.8°C) | 2.7°C (36.8°F)36.8°F (2.7°C) | 18.4 |
For ornamental alliums, also check drainage, sunlight and cultivar requirements. See the rainfall context above; humidity, irrigation and local pest pressure are not calculated.
Local growing check
Before choosing ornamental alliums for Berwyn
- May is the wettest month in the rainfall grid, averaging about 119 mm. Check how your own bed drains and plan watering around actual soil moisture; monthly rain totals are not an irrigation schedule.
The temperature and rainfall tables on this page show the underlying climate estimates. How these suitability checks work · Record what happens in your garden.
Before you plant ornamental alliums
Use the calendar to plan ahead, then check the conditions in your own garden.
- Frost tolerance does not make saturated or frozen soil suitable for planting. Wait until the bed can be worked without smearing or compacting it.
- Use the maturity range on your seed packet when choosing a variety. Cold, short days and transplant stress can make harvest later than the calendar estimate.
Plan for fading allium leaves beneath the flower heads
Use a sunny, drained site and follow depth guidance for the bulb size. Persistent wetness can cause rot, so a low waterlogged border is a poor choice despite a suitable calendar date.
Leave foliage to finish naturally and decide whether to retain dry seed heads for structure or remove them before seed spreads. Mark dormant bulbs to avoid later digging damage. Keep ornamental alliums labelled separately from edible onions and garlic.
What to record this season
Did companion planting hide fading leaves without shading or crowding the bulbs? Add the cultivar, sowing date and conditions to your growing record so you can compare next season.
Further reading: NC State Extension: Allium hollandicum . General crop guidance; local dates on this site are calculated separately. Source checked 19 September 2026 by Safe to Plant; this is not an expert review.
The reference describes plant identity and cultivation. Its regional calendar months and hardiness observations do not validate the calculated dates for another location.
How the dates are calculated and their limits · Keep a growing record · Report a local discrepancy
Why these dates
Use a sunny, drained site and follow depth guidance for the bulb size. Persistent wetness can cause rot, so a low waterlogged border is a poor choice despite a suitable calendar date.
Check the named variety and local forecast before exposing young growth. The catalog cold-damage figure is a reference, not a guarantee: plant age, acclimatisation, wind and the duration of a freeze affect damage.
Ornamental alliums are anchored to the first autumn frost rather than the last spring one, so the planting window on this page counts backwards from the end of the season in Berwyn instead of forwards from its start.
Berwyn is zone 7a, within the crop's general range. This is an initial cold-hardiness check; cultivar, exposure, drainage and unusual winters can still change the outcome.
The median frost-free season here is 227 days, against 180 to 230 days to first flower for this crop counted from sowing. That margin is what decides whether a late sowing is worth attempting.
Also in Berwyn
Other flowers to time
Growing ornamental alliums in Berwyn
Can I grow ornamental alliums in Berwyn?
The calendar estimates a planting window, not a guarantee of success. May is the wettest month in the rainfall grid, averaging about 119 mm. Check how your own bed drains and plan watering around actual soil moisture; monthly rain totals are not an irrigation schedule.
When should I plant ornamental alliums in Berwyn?
Aim for 6-20 Oct in an average year. If you would rather not gamble on the weather, wait until 22 Sep - 6 Oct - by then frost has finished in nine years out of ten.
When might ornamental alliums flower in Berwyn?
The estimated period is 1 May - 30 Jun, based on 180 to 230 days from the modeled planting date. Cultivar, planting material and growing conditions can change this interval; check the plant as well as the calendar.
How much frost will ornamental alliums tolerate?
Check the named variety and local forecast before exposing young growth. The catalog cold-damage figure is a reference, not a guarantee: plant age, acclimatisation, wind and the duration of a freeze affect damage.