What to Plant First in a Kentucky Greenhouse

The decision of what to plant first in a Kentucky greenhouse should be deliberate and timed to match local climate patterns, greenhouse capability, and your goals. Kentucky sits in USDA zones roughly 6a-7b depending on location, which means cold winters and warm, humid summers. A greenhouse offers the ability to extend seasons, jump-start seedlings, and raise tender crops that would otherwise struggle outdoors. This guide outlines the best first crops for a Kentucky greenhouse, practical schedules, soil and container recommendations, and step-by-step actions to maximize success in your first weeks.
Kentucky growers often want both early-season vegetables for spring markets and robust transplants for summer production. The right mix includes cool-season greens started early for harvest, warm-season crops started later as transplants, and fast-maturing herbs and seedlings for continual turnover. I provide concrete temperatures, sowing depths, timings, pot sizes, and care notes so you can act with confidence.

Kentucky greenhouse context: climate, seasons, and common constraints

A greenhouse changes the microclimate but does not erase the realities of hard winters, spring frosts, or humid summers. Understanding local dates and greenhouse limitations is the first step to choosing what to plant first.
Greenhouses in Kentucky are typically used to:

• Extend the growing season by 4-10 weeks in spring and fall.
• Protect seedlings from late frosts and cold snaps.
• Overwinter certain crops such as lettuce, herbs, and potted ornamentals.
• Provide a controlled environment for warm-season crops when heating and ventilation are available.

Common constraints to consider:

• Heating: If you do not have supplemental heat, the greenhouse will still drop near outdoor temperatures in winter nights.
• Ventilation: High daytime temperatures in spring and summer require shade, vents, or fans.
• Humidity: Kentucky’s humid climate increases fungal pressure; air circulation and spacing are important.

First goals: what you want from your greenhouse

Decide your primary objective before choosing crops. Typical goals include:

• Producing early-market greens and salads for personal use or sale.
• Raising vigorous tomato, pepper, and eggplant transplants for outdoor or greenhouse production.
• Growing year-round herbs and microgreens for culinary use or income.
• Overwintering cold-tolerant crops to harvest in late winter and early spring.

Your goals determine timing: leafy greens and herbs can be sown earliest; warm-season crop transplants are started later once the risk of frost recedes.

What to plant first: categories and timing

Start with crops that match the conditions you can provide and the calendar. Below I list recommended first plantings and explain why they make good early greenhouse crops in Kentucky.

• Cool-season greens (lettuce, spinach, arugula, mustard, tatsoi)
• Hardy brassicas for transplants (kale, collards, cabbage, broccoli)
• Early alliums (onion sets, shallots; onion seedlings started from seed indoors 8-12 weeks before transplant)
• Herbs (parsley, cilantro, chives; basil slightly later once temperatures are consistently warm)
• Fast microgreens and baby salad mixes
• Seedlings of warm-season crops started later: tomatoes, peppers, eggplants, cucumbers, squash

Timing basics for a Kentucky greenhouse

• Late January-February: Begin cool-season crops and brassica transplants if you have a heated or well-insulated greenhouse. Unheated greenhouse growers can start in mid-February to early March as sunny days lengthen.
• March-April: Start onion seed indoors in January-February for transplanting in March/April. Begin warm-season crop transplants (tomatoes/peppers) inside or in the greenhouse later in March or April depending on last frost.
• April-May: Harden off and transplant warm-season seedlings in May after the last frost; continue sowing successive rounds of greens.
• September-October: Begin fall/winter greens and overwintering crops for harvest into late winter.

Detailed crop recommendations and practical specifics

Below are crops that give the highest reliability and return on investment when planted first in a Kentucky greenhouse. For each crop I provide sowing depth, temperature range, pot size or spacing, light needs, and timing.

Lettuce and salad greens

• Sowing depth: 1/8″ to 1/4″.
• Soil temperature: 45-70degF; germination best at 60-70degF.
• Container/pot: Seed flats or 4″ pots; can be direct-sown in shallow trays.
• Light: Bright, diffused light; avoid scorching midday sun in a south-facing greenhouse.
• Timing: Sow starting late January (heated greenhouse) or mid-February (unheated). Stagger plantings every 2-3 weeks for continuous harvest.
• Notes: Use a loose seed-starting mix and keep evenly moist. Shade cloth may be needed when spring sun intensifies. Prevent damp-off by good air circulation.

Spinach and Swiss chard

• Sowing depth: 1/2″.
• Soil temperature: 40-75degF; germinates at cooler temps than many crops.
• Container: 4-6″ pots or in-ground beds.
• Timing: Start alongside lettuce in late winter to early spring.
• Notes: Spinach can bolt in hot weather; plan harvests before sustained warmth.

Kale, collards, broccoli, cabbage (brassicas)

• Sowing depth: 1/4″-1/2″.
• Temperature: 50-75degF for germination; cooler temps (55-65degF) produce sturdy seedlings.
• Container: 3-4″ pots initially, then 6-8″ for final transplant.
• Timing: Start 6-8 weeks before transplant date; in Kentucky start brassicas in late January to March in a greenhouse.
• Notes: These are excellent as greenhouse-hardened transplants; protect from flea beetles and cabbage worms after transplanting.

Onions and shallots

• Method: Start seed indoors or plant sets.
• Timing: Onion seed started in January-February for transplant in March, or plant sets in early spring.
• Depth/spacing: 1/4″ depth for seed; 4-6″ spacing.
• Notes: Long-day onion varieties are suited for Kentucky. Keep soil consistently moist but not waterlogged.

Fast herbs and microgreens

• Suitable herbs: Parsley, chives, cilantro (cool-season); basil later when temperatures are warm.
• Microgreens: Arugula, radish, broccoli, sunflower for rapid harvests (7-21 days).
• Timing: Microgreens can be sown anytime and are an excellent first project in an unheated greenhouse.

Tomatoes, peppers, eggplants (warm-season transplants)

• Start indoors or in heated greenhouse 6-8 weeks before last frost for peppers/eggplants, and 4-6 weeks for tomatoes if using vigorous varieties.
• Temperature: Night temps above 55degF ideal for peppers/eggplants; tomatoes are tolerant of slightly cooler nights but benefit from consistent warmth.
• Pot size: 4-6″ for initial growth; move to 1-3 gallon containers for final transplant.
• Timing: In Kentucky, plan to transplant outdoors or into a production greenhouse in May after the danger of frost has passed.

Potting mixes, fertilization, and watering

Use sterile, well-draining seed-starting mix for seedlings; when moving to larger pots switch to a mix with more compost or aged manure for nutrient retention.

• Basic seed-starting mix: fine peat or coconut coir, vermiculite, and perlite.
• Potting mix for transplants: 40% compost, 30% peat or coir, 30% perlite/vermiculite.
• Fertilization: Begin with a dilute balanced fertilizer (e.g., 10-10-10 at quarter strength) once true leaves appear; increase as plants grow to half strength.
• Watering: Bottom-water flats when possible to encourage strong roots; avoid overhead watering that raises humidity and fungal risk. Keep moisture even — not waterlogged.

Temperature, light, and humidity control

• Temperature targets: Keep daytime temps for cool-season crops in the greenhouse around 55-70degF and night temps no lower than 40-45degF for tender greens. Warm-season transplants need day temps of 70-75degF and nights above 55degF.
• Light: Provide 10-14 hours of light for most seedlings. Supplemental LEDs are useful in late winter for southern-facing greenhouses that still have short days.
• Humidity: Aim for 50-70% relative humidity. Ventilate in the morning to decrease humidity spikes; use fans to create horizontal airflow and reduce disease.

Pest and disease prevention

• Start clean: Sterilize trays and tools between uses to limit disease carryover.
• Inspect seedlings daily for aphids, fungus gnats, damping-off.
• Use sticky cards, beneficial insects, or organic treatments (insecticidal soap, Bacillus thuringiensis for caterpillars) as needed.
• Space seedlings and provide airflow to reduce fungal outbreaks; apply a copper or sulfur spray only as a preventive and following label directions.

Hardening off and transplanting

Hardening off is essential before moving greenhouse seedlings outdoors or into an unheated production greenhouse.

• Begin 7-10 days before transplanting: expose seedlings to outdoor conditions for a few hours a day, gradually increasing the duration and intensity of sunlight and wind exposure.
• Avoid sudden cold nights; always check weather forecasts — bring plants back inside if frost is predicted.
• Transplant on an overcast day or late afternoon to reduce transplant shock.

Succession planting and crop turnover

A greenhouse allows consecutive crops, staggered every 2-3 weeks for quick-turn greens, and every 6-8 weeks for longer-season seedlings. Plan a rotation:

• Week 0: Sow lettuce and microgreens.
• Week 2: Start brassicas.
• Week 4: Begin first round of tomato/pepper seeds indoors.
• Continue rotating to keep continuous production.

Practical checklist for your first greenhouse plantings

1. Decide your objective: early greens, transplants, or year-round herbs.
2. Assess greenhouse capabilities: heating, ventilation, and light.
3. Prepare sterile seed-starting mix and clean trays.
4. Sow cool-season greens first (late January-March depending on heat).
5. Start brassica and onion seeds 6-8 weeks before expected transplant.
6. Monitor temperature and humidity daily; ventilate and shade as needed.
7. Harden off seedlings before transplanting outside or to a production greenhouse.
8. Stagger sowings every 2-3 weeks to maintain supply.

Troubleshooting common problems

• Poor germination: Check seed age and storage, increase bottom heat slightly, ensure consistent moisture.
• Leggy seedlings: Increase light intensity or move lights closer; reduce temps slightly.
• Damping-off: Improve airflow, avoid over-watering, use sterile medium.
• Aphids or whiteflies: Introduce predators (ladybugs, lacewings) or use insecticidal soap.

Final takeaways

Start with cool-season greens and brassica transplants: they are forgiving, quick to produce, and give the fastest returns in a Kentucky greenhouse. Herbs and microgreens offer rapid turnover and income potential. Reserve warm-season crops for later seedings timed to your last frost date, and always harden off plants before moving them outdoors. With appropriate soil mixes, attention to temperature and humidity, and a schedule for succession planting, your greenhouse can produce months of reliable harvests and vigorous transplants for the growing season ahead.