When to plant Lilies in Boulder County, CO
Boulder County sits in cold Zone 6a. Plant Lilies May 21–June 4 for the single annual harvest; the September 26 first frost closes the window.
When to Plant Lilies in Boulder County, CO
Your July game plan for Boulder County, Colorado
Your Boulder County, Colorado garden is entering a new phase. Here's what's on the schedule for July and why each task matters now.
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Pick lilies
The more you pick, the more the plant produces. Letting fruit overripen tells the plant it's time to stop.
To set up a strong August, finish these tasks
- First harvests: lilies
Lilies (Lilium spp.) are among the most impressive summer-blooming bulbs, delivering bold, upward- or outward-facing trumpet blooms on tall stems from June through August. Asiatic hybrids are the most cold-hardy, earliest to bloom, and easiest to grow; Oriental hybrids bloom later with intensely fragrant, larger flowers. Orienpet (OT) hybrids combine the hardiness of Asiatics with the fragrance and size of Orientals. True lilies (not to be confused with daylilies, which are Hemerocallis) form scaly bulbs that persist and multiply underground, returning reliably each year with increasingly large clumps.
Boulder County, Colorado is in USDA Zone 6a. The average last spring frost is May 14 and the first fall frost is September 26, giving you a growing season of approximately 135 days.
At an elevation of 7,657 feet, Boulder County receives approximately 22.2 inches of rainfall annually with predominantly loam soil. Summer highs average 86°F, providing good warmth for Lilies during the growing season.
Boulder County Soil Profile
Soil Type
Loam
Soil pH
6.7-8
Drainage
Well Drained
Lilies Planting Risk Windows
Percentages indicate frost risk at transplant. The 70% safe window means there is a 30% chance of frost after transplant — suitable for cold-hardy crops or gardeners with frost protection. The 90% safe window is best for tender plants.
Soil Compatibility in Boulder County
How your county's soil matches Lilies's growing requirements.
Soil pH
Your soil pH (6.7–8.0) is more alkaline than Lilies prefers (6.0–6.5). Add sulfur or peat moss to lower pH.
Soil Texture
The loam soil in Boulder County is excellent for Lilies — good drainage, moisture retention, and nutrient holding capacity.
Drainage
Drainage is adequate for Lilies.
Organic Matter
Organic matter is moderate (2.3%). Annual compost additions will help Lilies.
How to Plant Lilies
Succession Planting Lilies
Sow every 8 weeks. Last sowing by May 29 to harvest before frost.
Lilies Water Budget
Water stress score is 6/10 — consider drought-tolerant varieties and mulching
Monthly Watering Guide for Lilies
Lilies needs approximately 1 inches of water per week (4.3" per month). Here's how your county's rainfall compares month by month.
| Month | Lilies Needs | Rainfall | You Supplement | Action |
|---|---|---|---|---|
| Jan | — | 1.7" | 0" | ❄️ Dormant |
| Feb | — | 1.3" | 0" | ❄️ Dormant |
| Mar | — | 1.8" | 0" | ❄️ Dormant |
| Apr | — | 2.2" | 0" | ❄️ Dormant |
| May | 4.3" | 3" | 1.3" | 💧 Light watering |
| Jun | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Jul | 4.3" | 2" | 2.3" | 🚿 Regular watering |
| Aug | 4.3" | 2.1" | 2.2" | 🚿 Regular watering |
| Sep | 4.3" | 2.1" | 2.2" | 🚿 Regular watering |
| Oct | — | 1.9" | 0" | ❄️ Dormant |
| Nov | — | 1.4" | 0" | ❄️ Dormant |
| Dec | — | 1.2" | 0" | ❄️ Dormant |
Water needs are for active growing months only (May–Sep in Boulder County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Lilies Heat Requirements (GDD)
What are Growing Degree Days (GDD)?
Growing Degree Days measure the total warmth your plants receive during the growing season. Think of it as a "heat bank" — every day above 50°F deposits warmth that helps your plants grow.
Each plant needs a certain amount of accumulated heat to mature. If your county provides more GDD than the plant needs, it's a great fit. If it's close, you may want to choose faster-maturing varieties or start seeds indoors to get a head start.
Lilies Planting Timeline — Boulder County, CO
Lilies Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Transplant Outdoors | May 21 | May 21 – Jun 4 |
| Bloom | July 30 | Jul 30 – Nov 5 |
Plant 5" deep · 12" apart · Rows 18" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | — |
| April | — |
| May | Transplant Outdoors |
| June | Transplant Outdoors |
| July | Bloom |
| August | Bloom |
| September | Bloom |
| October | Bloom |
| November | Bloom |
| December | — |
Growing Conditions
☀️ Sun
Full Sun (6-8+ hours)
💧 Water
1"/week · 2-3 times/week
📅 Days to Maturity
70–120 days
🧪 Soil pH
Needs 6–6.5 · Your soil: too_alkaline
🗺️ USDA Zone
Zone 6a
📆 Growing Season
135 days in Boulder County
Growing Tips for Lilies in Boulder County
Direct sow Lilies outdoors after May 14 in Boulder County when soil has warmed and frost danger has passed.
Boulder County receives only 22" of rain annually. Lilies needs consistent moisture — install drip irrigation or water deeply 2-3 times per week.
General growing tips
Plant bulbs in fall (preferred, September–October) or spring, setting them 3 times their diameter deep (typically 4–6 inches) with the pointed end up. Lilies need excellent drainage — they will rot in wet or heavy clay soils. Site with full sun on stems and blooms but cool, shaded soil at the base (ground cover or shallow-rooted annuals at their feet is ideal). Remove spent blooms but leave stems and foliage until they yellow naturally, as the bulb needs the foliage to photosynthesize and rebuild energy stores. Red lily beetle is a serious pest in the Northeast and Pacific Northwest; inspect plants regularly and hand-pick adults and larvae. Never plant Oriental or Asiatic lilies near cats — all Lilium species are highly toxic to cats. Year 2+ bulbs produce the most stems and largest blooms.
Companion Planting
Good Companions
Check more plant combinations with our Companion Planting Checker →
Lilies in Other Locations
Frequently Asked Questions
When should I plant Lilies in Boulder County, CO?
Boulder County is in Zone 6a with an average last frost of May 14. Plan your Lilies planting based on this frost date — see the calendar above for exact timing.
What planting zone is Boulder County, CO?
Boulder County, Colorado is in USDA Hardiness Zone 6a. The average last spring frost is May 14 and first fall frost is September 26.
When should I plant Lilies in Boulder County, CO?
In Boulder County, CO, plant Lilies after the last frost (around May 14) and before the first frost (around September 26). Best results when soil holds above 50°F.
What growing zone is Boulder County, CO for Lilies?
Boulder County sits in USDA Zone 6a. Lilies grows reliably in zones 3a through 9a, so it's a good fit here.
Can Lilies grow in Boulder County's climate?
Yes — Lilies grows well in Boulder County's temperate climate. Boulder County averages a 135-day frost-free season, with last frost around May 14 and first frost around September 26.
Your Boulder County Garden Planner — Free
A 22-page printable planner built for Boulder County (Zone 6a). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.