What To Plant First In A Colorado Greenhouse In Early Spring

Early spring in Colorado is a time of opportunity and caution. A greenhouse extends the already short growing season, protects young plants from late frosts, and lets you begin a full season of vegetable and herb production earlier than outdoor beds allow. But Colorado’s unique combination of altitude, intense sunlight, low humidity, and unpredictable late freezes means the wrong choices or poor greenhouse management can waste time and seedlings. This guide explains what to plant first in a Colorado greenhouse, when to plant it, and how to manage environmental factors so those early plantings thrive.

Understand Colorado climate constraints

Colorado is not one climate. Elevation ranges and USDA hardiness zones vary widely across the state, from valley plains to montane zones. Two constant factors are especially important for greenhouse growers:

• Strong solar radiation. High-altitude sun is intense and can rapidly heat a greenhouse on sunny days and cause sunscald on tender seedlings.
• Wide diurnal temperature swings and late frosts. Nights can be much colder than days well into spring.

These elements change how and when you start seeds and move transplants out of the greenhouse. Successful early-spring greenhouse planting balances warmth for germination and growth with protection from cold nights and sudden temperature swings.

Altitude and last frost dates

Know your average last frost date and adjust seed-start timing to it. In Denver and Front Range urban areas, the last average frost is often mid to late May. In higher mountain towns you may not be safe until June or even July. Use last frost as the anchor for scheduling.

Light and shade management

Seedlings need bright light, but too-intense midday sun in a small, unventilated greenhouse can burn tender leaves and stress plants. Plan for shade cloth and adjustable ventilation starting as soon as day temperatures exceed 70 to 80 F.

What to plant first: prioritized list and specifics

Early spring priority goes to crops that tolerate cool soils and rapid, early harvests. These give quick returns and let you clear space for warm-season crops later.

• Lettuce and mixed salad greens: arugula, mizuna, baby leaf mixes.
• Spinach and Swiss chard.
• Radishes.
• Peas (sugar snap and shelling).
• Kale and mustard greens.
• Brassicas for transplants: broccoli, cabbage, cauliflower.
• Onions and leeks (from sets or transplants).
• Early potatoes (chitted seed potatoes started in greenhouse).
• Herbs that tolerate cool weather: cilantro, parsley, chives, dill.
• Microgreens and quick sprouts for immediate harvest.

Below are crop-specific recommendations with sowing details and practical takeaways.

Leafy greens: quickest, most reliable payback

Lettuce, arugula, spinach, and other salad greens germinate well in cooler soil (40 to 70 F) and tolerate cool nights. Sow seed in shallow flats or narrow troughs and keep surface moist. Typical sowing depth is 1/8 to 1/4 inch for lettuce and most salad greens. Expect harvestable baby greens in 20 to 35 days when started in a greenhouse.
Practical takeaways:

• Sow successive batches every 10 to 14 days for continuous harvest.
• Thin or harvest as cut-and-come-again to maximize space.
• Avoid high nitrogen later if you want slower, denser heads for full-size lettuce.

Spinach, chard, and brassica greens

Spinach tolerates temperatures down to near freezing for short periods and will bolt when stressed by heat, so early spring greenhouse start gives the best quality leaves. Swiss chard is slightly more heat-tolerant and can be left to mature into summer in many Colorado microclimates.
Practical takeaways:

• Sow spinach 6 to 8 weeks before your last frost for early spring harvest.
• Provide some shade if daytime greenhouse temps exceed 70 to 75 F.

Radishes and other root crops

Radishes are a perfect early greenhouse crop because they germinate and mature quickly, often in 20 to 30 days. Sow a bit deeper than salad greens, about 1/2 inch, in loose, well-drained mix.
Practical takeaways:

• Use deep trays or raised beds for root development.
• Keep soil consistently moist to avoid woody roots.

Peas

Peas can be sown in the greenhouse several weeks before the last frost; they prefer cool conditions and will root and climb as soon as soils warm. Sow 1 to 1.5 inches deep, provide trellises or supports, and transplant or direct sow into a protected outdoor bed when nighttime temps stabilize above freezing.
Practical takeaways:

• Start peas 4 to 6 weeks before last frost to get early harvests.
• Harden off gradually before transplanting outdoors.

Brassicas for transplants

Broccoli, cauliflower, cabbage, and Brussels sprouts are best started in the greenhouse as transplants 6 to 8 weeks before the last frost. They tolerate cool weather and can be set out early to establish before summer heat.
Practical takeaways:

• Use deep cells to encourage strong root systems.
• Fertilize transplants moderately; avoid excessive stretch.

Warm-season crops: start seeds early but do not move out too soon

Tomatoes, peppers, and eggplants need warm soil and air. Start seeds in the greenhouse 6 to 10 weeks before your last frost so transplants are sturdy when it is safe to move them outdoors. However, do not plant them outside until night temperatures consistently stay above 50 to 55 F for tomatoes and 60 to 65 F for peppers.
Practical takeaways:

• Use bottom heat or warm germination area to speed pepper and tomato germination (70 to 85 F preferred).
• Harden off seedlings gradually over 7 to 14 days before transplanting.

Seeds vs transplants: which to start in the greenhouse

Starting everything from seed in the greenhouse works, but consider time, space, and cost.

• Start high-value slow-growers from seed in the greenhouse (tomatoes, peppers, brassicas).
• Direct sow very early peas and hardy root crops in protected beds or use greenhouse as a cold frame only.
• Buy onion sets or transplants if you want reliable early onion yields without the 12-14 week seed start.

Greenhouse environment management for early spring

Managing temperature, light, humidity, and irrigation is as important as seed choice. Practical control steps follow.

Temperature control

• Target daytime greenhouse temps between 60 and 75 F for cool-season crops.
• Nighttime temps for cool greens can be 40 to 50 F; use row covers or small heaters if expected to drop below plant tolerance.
• Use a small thermostat-controlled heater for extreme cold snaps. Bottom heat mats help germination for tomatoes and peppers; avoid leaving mats on for hardened seedlings.

Ventilation and shading

• Vent when internal temps rise above 75 to 80 F to prevent overheating.
• Install a 30 to 50 percent shade cloth to reduce midday heat and prevent sunscald on young leaves once day lengths and sun intensity increase.

Soil, containers, and fertility

• Use a sterile seed-starting mix for flats; transplant to a heavier potting or greenhouse mix with added compost once true leaves form.
• Keep fertility low for early baby greens to avoid excessive stretch; switch to balanced liquid feed once transplants have 2 to 3 true leaves.

Water and humidity

• Water from the bottom when possible to encourage strong roots and reduce damping-off risk.
• Maintain moderate humidity inside the greenhouse; too high encourages fungal disease, too low causes stress. Aim for relative humidity around 40 to 60 percent.

Pest prevention

• Monitor for aphids, whiteflies, and fungus gnats. Early spring greenhouse crops are less pest-prone but still vulnerable.
• Use sticky traps, beneficial insects, or light neem/soap sprays as needed. Keep sanitation high: remove decaying leaves, avoid overwatering, and disinfect trays between batches.

Practical planting checklist

1. Know your last frost date and create a planting calendar counting back weeks for each crop.
2. Prepare seed mixes, trays, and sterile starting media before the first sowing.
3. Sow quick crops first: lettuce, arugula, spinach, radish, and peas as early as 8 weeks before last frost for peas and 4 to 6 weeks for leafy greens.
4. Start brassica and tomato/pepper seedlings 6 to 10 weeks before your last frost, using bottom heat for warmth-sensitive seeds.
5. Monitor greenhouse temps daily and plan ventilation and shade deployment in advance.
6. Harden off transplants gradually for 7 to 14 days before moving to outdoor beds.

Sample timeline examples (approximate)

• Typical Denver area (zone 5b to 6a, last frost mid-May): start lettuce and spinach in late March to early April; peas and radishes mid-March to late March; broccoli and cabbage seedlings mid-March to early April; tomato seeds in late March to early April for transplanting after mid-May.
• Higher elevation (zone 3 to 4, last frost late May to early June): delay warm-season starts; begin salad greens and peas in late April inside greenhouse; start brassica transplants in early to mid-May; begin tomatoes in late April only if you have reliable greenhouse heat.

Final practical takeaways

• Prioritize cool-season greens, radishes, peas, and brassicas first in a Colorado greenhouse. They tolerate cool soils and give fast returns.
• Start warm-season crops indoors early enough to be ready when frost danger passes, but do not rush transplanting them outside.
• Pay as much attention to greenhouse management as you do to planting dates: temperature control, ventilation, shading, and watering determine success for early spring starts.
• Keep an ongoing sowing schedule for salad greens and a clear hardening-off routine for transplants. With good timing and vigilant greenhouse management, you can gain several weeks of productive harvest in Colorado’s challenging spring climate.