Now
30°C · Partly cloudy
Today
30° / 17°
Soil 6 cm
25°C
First frost
~32 days (Oct 10)
Gardening in Denver
AI field guide · refreshed weeklyDenver Gardening: Early September—Sow Cool-Season Crops, Harvest Summer Bounty
You're in zone 5a with 32 days until your average first frost (Oct 10). Soil at 25°C (77°F) is still warm enough for quick-germinating cool-season crops; nighttime temps dipping into the low teens this week signal the shift. Plant now for a 4–6 week harvest window before hard freeze.
Sow outdoors now
Spinach (Space, Bloomsdale)
Direct sow 1 cm deep, 10 cm apart. Germinates in 7–10 days at soil temp 25°C; ready to harvest in 35–40 days, well before frost.
Lettuce (Buttercrunch, Oakleaf)
Sow 0.5 cm deep, thin to 15 cm spacing. Tolerates light frost; succession-sow every 10 days through mid-September for continuous harvest.
Radish (Cherry Belle, French Breakfast)
Direct sow 1 cm deep, 5 cm apart. Ready in 25–30 days; cold actually improves flavor and crispness.
Arugula (Roquette)
Sow 0.5 cm deep, 10 cm apart. Germinates fast in warm soil; peppery leaves ready in 30–35 days, sweeter after light frost.
Peas (Sugar Snap, Oregon Sugar Pod)
Soak seed 4 hours, sow 2 cm deep, 5 cm apart along trellis. Slow to germinate in warm soil but will flower before frost if started now.
Start indoors now
Kale (Winterbor, Lacinato)
Start in 6-cell packs; transplant outdoors in 3–4 weeks for 6-week growth before frost. Kale sweetens after frost and overwinters in zone 5a.
Cabbage (Red Acre, Savoy)
Start indoors now for transplant in late September; heads form slowly in cool weather but mature by late October.
Broccoli (Calabrese, De Cicco)
Start indoors for transplant in 3 weeks; needs 50–70 days to head, so timing is tight but doable before hard freeze.
Harvest now
Tomatoes (all varieties)
Pick all remaining fruit, including green ones; ripen indoors at room temp or use in salsa/fried green tomatoes before frost arrives.
Peppers (Bell, Jalapeño)
Harvest mature peppers now; plants will stop producing as nights cool. Leave immature fruit to ripen indoors if desired.
Beans (Bush, Pole)
Pick regularly to encourage final flush; frost will kill plants within 2–3 weeks, so harvest all edible pods.
Summer Squash & Zucchini
Cut daily while tender; plants decline rapidly as temps drop. Remove any diseased or damaged fruit to redirect energy.
Tasks this week
- Water deeply 2–3 times this week—low humidity (24%) and warm days pull moisture fast; cool nights mean less evaporation, so monitor soil 5 cm down.
- Deadhead spent flowers on perennials (coneflower, black-eyed Susan) to extend bloom and prevent self-seeding; leave some seed heads for birds.
- Scout for spider mites and aphids on remaining warm-season crops; spray with neem oil in early morning if populations spike.
- Mulch newly sown beds with 2–3 cm straw to retain moisture and moderate soil temp swings; remove mulch if frost threatens to let ground warm.
- Cut back ornamental grasses and perennials only after frost blackens foliage; early cutting invites regrowth that frost will damage.
Watch out for
- Hail and wind: Forecast shows gusts to 30 km/h (19 mph) mid-week; stake tall crops and cover tender seedlings if severe weather develops.
- Frost creep: First frost can arrive 1–2 weeks early in Denver microclimates (low-lying yards, north-facing slopes). Have frost cloth ready after Sept 20.
- Powdery mildew: Cool nights + warm days + low humidity create ideal conditions; ensure air circulation and avoid overhead watering.
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