A greenhouse allows a gardener to modify the growing environment, providing protection from harsh external elements and regulating temperature. This controlled space enables the cultivation of plants outside their normal season, significantly extending the potential harvest window. Determining the correct time to start planting inside a greenhouse is highly flexible, but it requires careful planning anchored to specific environmental data. This process depends entirely on understanding both the outdoor climate and the internal conditions of the structure.
Determining Your Baseline Timing
Calculating the average last expected spring frost date for your location is the foundational step in determining the earliest safe planting time. This date is an average based on historical climate data and signifies the point after which the risk of a freeze, a temperature drop to 32°F (0°C), significantly decreases. Using this external factor as an anchor point is necessary even with a greenhouse, as most plants will eventually be moved outside or rely on the greenhouse’s ability to buffer the worst of the cold.
The local USDA Hardiness Zone classifies regions based on the average annual extreme minimum winter temperature. While primarily used for selecting perennial plants, it offers context for the severity of your winter and the length of your growing season. This context influences how much you must rely on the greenhouse’s heating capability, but the most immediate constraint is the plant-specific requirement for internal greenhouse temperature, especially the minimum night temperature.
Monitoring the internal temperature is crucial because plant growth stalls or ceases below a certain minimum temperature. Most common greenhouse vegetables, such as tomatoes and peppers, thrive when the night temperature remains between 55°F and 65°F (13°C to 18°C). Hardy cool-season crops, like cabbage and broccoli, can tolerate much cooler night temperatures, sometimes as low as 45°F (7°C). The decision to plant early must align with your ability to consistently maintain the required minimum nighttime temperature for the specific crop you are growing.
Scheduling Seed Starts
The baseline timing established by the last frost date is used to calculate the precise indoor starting time for seeds intended for later outdoor transplanting. This calculation is known as the “count-back” method, which ensures seedlings are the optimal size and age when the outdoor conditions are finally safe for planting. A seed packet typically lists the number of weeks required for a seedling to reach transplant size, and this number is counted backward from the target outdoor transplant date.
The timing differs significantly between warm-season and cool-season crops due to their varying cold tolerance. Warm-season plants, including tomatoes, peppers, and eggplants, are highly sensitive to frost and should not be transplanted until one to two weeks after the average last frost date, when the soil and air are reliably warm. For example, if the target outdoor transplant date is May 15th, and a tomato variety takes eight weeks to reach transplant size, the seeds should be started indoors around March 20th.
Cool-season crops, such as cabbage, broccoli, and Brussels sprouts, are frost-hardy and can withstand cooler temperatures. These plants can often be transplanted outdoors several weeks before the last expected frost date, allowing for a much earlier start in the garden. Starting these seeds too early, however, can result in overgrown, stressed seedlings that perform poorly when moved to the garden.
Utilizing the Greenhouse for Year-Round Production
Beyond starting seeds, the greenhouse can be used for continuous cropping and extending the harvest of mature plants well into the cooler months. This involves planting cold-hardy crops directly into the greenhouse soil or beds during late summer or early fall. Leafy greens like spinach, kale, and lettuce, along with root vegetables such as carrots and beets, are excellent choices for fall and winter production because they tolerate lower temperatures and shorter daylight hours.
The goal for deep winter production is to have the plants nearly mature before the shortest days of the year, typically from November through January, when growth slows significantly. During this period, the plants enter a semi-dormant state, and harvests are taken slowly from the existing foliage.
In regions with limited winter sunlight, supplemental lighting may be necessary to maintain growth and prevent the plants from stalling completely. High-pressure sodium (HPS) or LED lights can provide the necessary daily light integral (DLI) to support photosynthesis during cloudy days or short winter hours. In colder climates, supplemental heat may also be required to keep the internal temperature above freezing, though many cold-hardy crops sweeten in flavor when exposed to near-freezing temperatures.