when to start seeds in greenhouse

When Starting My Seeds, Do I Need to Heat the Greenhouse?

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Like most other kinds of babies, seeds and young plants need warmth to coax them to grow. Although a greenhouse may feel nice and toasty during the day even during the winter, its nighttime temperatures can doom germinating seeds. The seeds of some plant species perform well in cool temperatures while others flourish only when it’s decidedly warmer. A number of factors must be considered before deciding how, and to what degree, to add heat when starting seeds in a greenhouse.

Know the Temp for Seedlings

Although the seeds of some plant species require a temperature as low as 50 degrees Fahrenheit to germinate, the optimal temperature for seedlings is between 65 and 75 degrees Fahrenheit, The Old Farmer’s Almanac says. Most seeds can and will sprout at temperatures higher or lower than their optimal germination temperatures, though extremes in either direction greatly diminish germination rates. The majority of greenhouses can easily reach optimal temperatures during the day even in winter, eliminating the need for supplemental heating during daylight hours.

Plan Now for a “Blanket”

A sunny winter day feels scandalously tropical in a greenhouse, but come nighttime, that warmth rapidly evaporates, leaving seeds and the young plants they create dangerously exposed. In the cold of night, even a high-quality, unheated greenhouse eventually is only a few degrees above the air temperature surrounding it.

Germinating seeds and seedlings need a “blanket” to keep them warm in winter, even in a greenhouse. That blanket often comes in the form of a supplemental heat source. After seed germination, many kinds of vegetable and perennial seedlings need temperatures of at least 50 degrees Fahrenheit to continue developing properly.

Raise the Temp for Your Seedlings

Depending on your greenhouse’s size and how much you are willing to invest in supplemental heating, providing heat adequate for seed germination and seedling subsistence is as simple as plugging in a heat mat or as involved as installing a forced-air duct distribution system. Heat mats are relatively inexpensive and flexible but are best suited for very small-scale growers.

Many greenhouse hobbyists use propane heat distributed via fans to the greenhouse interior while larger growers may invest in a duct system installed near the greenhouse roof line to distribute hot air evenly throughout the structure, Ferrell Gas says. Electric space heaters are another small-scale, economical way to raise the temperature for seedlings inside a cold greenhouse.

Ventilate Your Seedlings

Although some supplemental heating may be necessary during daylight hours in the coldest months, interior greenhouse temperatures usually climb rapidly when the sun rises. Seedlings growing in an overly warm environment develop too rapidly and become spindly and weak, leading to shoddy plants of poor transplant quality.

Given the risk, almost every grower needs to provide a way for some hot air to escape the greenhouse to keep its internal temperature about 10 to 15 degrees Fahrenheit above its nighttime temperature. A louvered window or moveable panel in the greenhouse’s roof is a common way to vent excess heat, though a cracked door or oscillating fan can also reduce the temperature to an acceptable level.

When Starting My Seeds, Do I Need to Heat the Greenhouse?. Like most other kinds of babies, seeds and young plants need warmth to coax them to grow. Although a greenhouse may feel nice and toasty during the day even in the winter months, its nighttime temperatures can doom germinating seeds. Some plant species’ …

How to Use Greenhouses to Grow Plants From Seeds

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Though the weather outside might be frightful, you can start your garden early with a greenhouse. Whether you use a cold frame, mini-greenhouse or freestanding unit, your seeds germinate in a warm, comfortable environment that fosters growth. By the time spring arrives, your seedlings will be sturdy transplants, ready for the garden.

Cold Frame

In its simplest form, a cold frame is a shallow pit, often framed in wood, and covered with a glass-topped lid. Generally, the north, or back, side of the frame is taller than the south side. This allows the sun’s rays to penetrate into the interior and warm the seed trays, peat pots or flowerpots. While the temperatures inside a cold frame may only rise to 10 degrees Fahrenheit above the outside temperature, it is enough for cold-tolerant vegetable or flower seeds to sprout. Broccoli, cabbage, peas and radishes are among the seeds that germinate in temperatures as low as 40 degrees, although the optimum temperature ranges from 75 to 85 degrees.

Hotbed Frame

A cold frame may be heated with manure, heating coils or a light bulb, making it a hotbed. To use manure, dig the pit 30 inches deeper than you’d dig a hole for a cold frame. Add 6 inches of coarse gravel to the pit and tamp it. Fill the bottom of the hole with 18 inches of partially composted manure, such as horse manure. Cover the manure with 4 inches of potting soil. Place the lid onto the hotbed and wait until the soil temperature drops to 85 degrees before adding seeds or seed trays to the hotbed. Monitor the soil temperature, and lift the lid if it rises above 85 degrees. If you use heating elements or a light fixture, always plug the unit into a GFCI-protected outlet.

Indoor Mini-Greenhouse

An indoor mini-greenhouse is helpful when your home is drafty or its temperatures are lower than the optimum germination temperature for your seeds. Tomatoes and peppers germinate best between 80 and 85 degrees. While there are commercially available mini-greenhouses, you can use a simple clear plastic container, such as one that strawberries, cherry tomatoes or other fresh fruits are packed in at the supermarket. The clear lid serves as the “greenhouse.” Alternately, you can use any clear container and cover it with plastic wrap to make your own mini-greenhouse.

Freestanding Greenhouse

A freestanding greenhouse allows you to start multiple seeds on shelves or benches, depending on the size of the unit. Add heaters and fans to regulate the interior temperature. Fluorescent lighting adds more light on cloudy days to encourage seedling growth. Before starting the seeds, disinfect the shelves, benches, pots and trays; the warm, moist environment inside the greenhouse makes an ideal climate for algae, fungi, gnats and other pests, as well as your plants.

Seed Starting

Fill peat pots, flowerpots or seed-starting trays with moist seed-starting mix. Plant the seeds according to the seed packet directions, usually two or three seeds per pot at a depth ranging from 1/8 to 1/2 inch. Cover the seeds with soil and mist with water. Cover the pots or trays with plastic wrap. Place them in a warm, brightly lit location. An electric heating mat manufactured for seed starting allows you to regulate the soil temperature. When the seedlings appear, remove the plastic wrap.

Continue to keep the planting mix moist, but not waterlogged. Snip the excess seedlings with scissors; leave one seedling per pot. Fertilize every one to two weeks with a one-quarter strength 5-10-5 water-soluble fertilizer. Water each seedling with 1/4 cup of the fertilizer solution. Water the plants after fertilizing. When the seedlings are approximately eight weeks old, they’re ready to harden off and plant outdoors, weather permitting. Otherwise, transplant them into larger containers so they can continue to grow.

How to Use Greenhouses to Grow Plants From Seeds. In nature, seeds and plants are subject to ever-changing temperatures and weather conditions. For gardeners attempting to grow finicky or troublesome plants from seed, these fluctuating conditions often cause frustration. Many plants have particular requirements before …