When To Start Seeds In A Texas Greenhouse

Growing from seed in a Texas greenhouse gives you powerful control over timing, temperature, and humidity — and that control is the difference between a bumper crop and wasted seed. Texas spans many climates, from the cool Panhandle to the humid Gulf Coast and the subtropical Rio Grande Valley. This article explains when to start seeds in a Texas greenhouse by region and crop type, gives practical temperature and timing targets, and offers step-by-step greenhouse-specific practices to maximize germination, seedling vigor, and transplant success.

Understand Texas climate zones and your last/first frost dates

The single most important datum for timing seed starting is your average last spring frost date and first fall frost date. Texas covers USDA hardiness zones roughly from zone 6 in the Panhandle into zone 10 along the southern coast. Never rely on statewide rules; use your local average frost dates or the historical records for your nearest town.
Broad regional guidance:

Panhandle and High Plains (Amarillo, Lubbock): last spring frost commonly mid- to late-May; first fall frost often in October.
North Texas (Dallas-Fort Worth, Wichita Falls): last spring frost mid-April to early April; first fall frost in late October to November.
Central Texas (Austin, San Antonio, Hill Country): last spring frost mid-March to early April; first fall frost typically in late November.
Gulf Coast and Houston area: last spring frost February-March; first fall frost November-December; mild winters.
South Texas and Lower Rio Grande Valley (McAllen, Brownsville): rare frosts; you can often grow year-round but watch for occasional cold snaps in winter.

Use those local dates to schedule seed starting. In a greenhouse you can start earlier than outdoor transplant dates, but you must manage heat and light accordingly.

Basic timing rules: weeks before transplant or expected planting

The general rule is to count backward from the date you plan to transplant seedlings outdoors (or the date you want a harvest to start):

Fast-maturing or small seedlings (leafy greens): start 4-6 weeks before transplant.
Brassicas (cabbage, broccoli, kale): start 6-8 weeks before transplant.
Tomatoes: start 6-8 weeks before transplant into the garden; start 8-10 weeks if you want larger transplants or are growing indeterminate varieties.
Peppers: start 8-10 weeks before transplant; peppers are slow to germinate and grow slowly.
Eggplants: start 8-10 weeks before transplant.
Cucurbits (cucumber, melon, squash): either direct sow outdoors after last frost or start 2-3 weeks before last frost in cell packs to minimize root disturbance.
Beans: generally direct sow; avoid transplanting unless using larger containers.
Herbs: basil 4-6 weeks; parsley and cilantro 4-8 weeks depending on desired size and variety.

These are starting points; adjust by region. For example, in the Panhandle start tomato seeds 8-10 weeks before the early June transplant window (so late March to early April), whereas on the Gulf Coast you can start tomatoes in late January to February for early spring transplanting.

Region-specific seed-starting calendars

Below are practical month windows by region for common categories. These assume you will transplant outdoors after the typical last frost for the region.

Panhandle and High Plains

Tomatoes and eggplants: start seeds late March to mid-April (for May transplanting).
Peppers: start seeds mid-March to early April.
Brassicas for spring: start late February to March.
Cool-season crops for fall: start seeds late July to August for transplanting in September.

North Texas (Dallas-Fort Worth)

Tomatoes: start seeds late February to mid-March (transplant mid- to late April).
Peppers and eggplant: start late February to early March.
Cucurbits: start in greenhouse mid-March if transplanting early, or direct sow mid-April.
Fall brassicas: start seeds in late July for August transplants or staggered succession.

Central Texas (Austin, San Antonio)

Tomatoes: start seeds in late January to early February for early spring transplants.
Peppers: start in mid-January to February.
Cucurbits: start greenhouse seeds in February to transplant in March, or direct sow in March.
Winter greens: start in October for fall planting.

Gulf Coast (Houston, Corpus Christi)

Tomatoes: start seeds in January to transplant in February-March.
Peppers: start in January.
Winter greens: start in September-October; many greens grow well through winter.

South Texas and Lower Rio Grande Valley

Many crops can be started nearly year-round. For spring/summer crops start seeds in December-February to transplant in late winter or early spring to avoid extreme summer heat.

Adjust these windows for microclimates, elevation, and your exact frost dates.

Temperature and light targets for germination and seedling growth

Greenhouses give you the power to hit the specific temperature ranges seeds need. Key targets:

Soil/seed zone temperature for germination:
Tomatoes: 70-85 F (21-29 C).
Peppers and eggplant: 75-90 F (24-32 C).
Lettuce and leafy greens: 60-70 F (16-21 C).
Brassicas: 65-75 F (18-24 C).
Cucurbits (cucumber, squash): 70-95 F (21-35 C).
Seedling air temperatures after germination:
Day: 65-75 F (18-24 C).
Night: 55-65 F (13-18 C).
Light: seedlings need bright, diffuse light. Provide 12-16 hours of supplemental light in winter or during cloudy spells to prevent legginess. Keep lights 2-4 inches above seedlings and raise as they grow.
Humidity and ventilation: maintain 50-70 percent relative humidity for germination, then reduce to 40-60 percent for sturdy growth. Ventilate to prevent overheating and fungal disease.

Heat mats, thermostats, and accurate soil thermometers are excellent investments for a Texas greenhouse.

Seed-starting mix, container choices, and sowing depth

Use a sterile, soilless seed-starting mix with good drainage. Avoid heavy garden soil in the greenhouse trays.

Containers: 72-cell trays for small seedlings, 4-inch pots for sturdy transplants, and single-cell plugs for species sensitive to root disturbance.
Sowing depth: follow packet directions, usually 2-3 times the seed width. Tiny seeds like lettuce or celery should be barely covered or surface-sown with light fine vermiculite on top.
Moisture: keep mixes evenly moist but not waterlogged. Use bottom-watering or fine misting to avoid displacing seeds.

Hardening off and transplant timing

Even in Texas, greenhouse-grown plants need hardening off unless they will live full-time in the greenhouse. Gradually expose seedlings to outdoor conditions over 7-14 days, increasing sun, wind, and temperature swings. In Texas summer, hardening is essential to avoid transplant shock and leaf burn.
Transplant when seedlings:

Have 2-4 true leaves (for small transplants) or are well rooted but not root-bound.
Soil temperatures outside match the crop needs (e.g., tomatoes prefer outdoor soil consistently above 60-65 F).
Night temperatures have stabilized above crop minimums for tender plants.

Common problems and practical fixes

Leggy seedlings: increase light intensity or duration, lower night temperature slightly, and avoid excessive fertilizer. Thin or transplant deeper to encourage strong stems.
Poor germination: check seed viability and soil temperature. Many seeds fail because the seed zone is too cool or too moist.
Damping-off: keep sanitation high, use sterile mix, avoid overhead watering, provide airflow, and remove affected seedlings immediately.
Overheating in summer: install shade cloth (30-50 percent) and provide forced ventilation or evaporative cooling to keep temps from soaring above 95 F, which stresses seedlings.
Root-bound plants: pot up to the next size container before transplant to avoid shock.

Practical greenhouse schedule and checklist

Determine your local last spring frost and first fall frost dates.
Choose target transplant dates and count backward using the crop-specific week guidelines above.
Prepare sterile trays and mix; test soil temperature with a probe.
Sow seeds at correct depth and label trays.
Maintain seed-zone and air temperature with heat mats and thermostats.
Provide 12-16 hours of light for winter starts; adjust lights as seedlings grow.
Begin fertilizing with a weak solution after first true leaves emerge.
Pot up as needed and harden off 7-14 days before transplant.
Transplant when outdoor soil and night temperatures are suitable for the crop.

Final practical takeaways

Base timing on your local frost dates, not a statewide calendar; Texas is too big for one rule.
Use the greenhouse to start tender crops earlier than outdoors, but manage heat and light carefully — overheating and low light cause more failures than being late.
Follow crop-specific weeks-before-transplant rules: peppers and eggplants need more lead time; cucurbits often do fine started later or direct-sown.
Monitor soil temperature, not just air temperature. A soil thermometer gives actionable data for germination decisions.
Hardening off is non-negotiable if moving seedlings outside, even in mild Texas climates.

Starting seeds in a Texas greenhouse is a balancing act between calendar dates, targeted soil and air temperatures, and crop-specific needs. With a small set of tools — soil thermometer, heat mat, good lighting, and a greenhouse ventilation plan — you can extend seasons, get early harvests, and turn seed packets into healthy, vigorous transplants that thrive once they hit Texas soil.