Greenhouse gardening: When is the best time to start planting?

Seed catalogs and garden magazines are full of the promise of spring and summer, especially this time of the year. And if you’ve dropped by your local hardware or grocery store recently, you probably saw gardening tools and seed packets on display as soon as you walked in the door.

Whether you’re a part-time gardener with one greenhouse or a fulltime grower with dozens, we know your green thumb is itching to dig into the soil and get those seeds in the ground. But when is the best time to start planting?

Getting started

As any gardener knows, there’s a lot of prep work that must happen before you can begin planting. Before starting seeds, it’s important to disinfect shelves, benches, pots, and trays. While the warm, humid atmosphere of your greenhouse makes it the ideal environment to start seedlings, it’s also the perfect climate for fungi, algae, gnats, and other pests to grow.

Regulating the humidity in your greenhouse with proper ventilation is one way to fight against these disease-causing organisms throughout the year.

As you prepare for another growing season, you’ll also want to keep soil health in mind. If you’re a producer who plants one crop continuously without rotation and are experiencing lower yields than you’d like to see, you may want to consider crop rotation.

When the same crop is planted year after year, the soil can have a buildup of diseases, such as bacterial wilt, bacterial canker, fusarium, and verticillium wilts. These can be remedied by rotating crops and treating the soil with organic or inorganic fertilizers to replenish soil fertility. (For example, you could grow onions or cauliflower this year if you grew tomatoes in the same spot last year.)

You can also combat diseased soil by bringing in fresh, quality soil from outside your greenhouse. While this is easier to do if you’ve grown in pots, it can still be done in larger garden beds.

Although a clean greenhouse and healthy soil are two very important pieces of the puzzle, your greenhouse wouldn’t be much to boast about without plants. As you begin thinking about what seeds you’ll sow and when, here are a few helpful tips to keep in mind:

• Be seed savvy. Start by sorting your seed packets into categories: cool season plants vs. warm season plants and those that grow best when directly sown into the soil vs. those that grow best when started indoors.
• Create a calendar. Find the average last frost date in your area (you can do this by searching your ZIP code online) and count backward six to eight weeks, depending on the directions on the back of your seed packet.
• Go soilless. Of course, you’ll need soil for your plants to grow, but if you’re starting seeds in pots or trays be sure to choose a soil mix designed for seed starting. Soilless mixes are designed for better drainage and to fight against soil-borne diseases. Regular potting soil or plain garden soil are often too heavy, so it’s recommended that you use a compost-based mix.
• Don’t forget the labels. When little green seedlings are peeking up from fresh dirt, they all look alike. It will be difficult to tell one sprouting seed from the next, so make sure to label each kind. That way you can follow the proper guidelines each plant needs for water, sunlight, and fertilizer.

Planting seeds

The good news about greenhouses is that they allow you to jumpstart the growing season, and using propane heaters to keep them warm, they help protect against late frosts to produce healthy seedlings. The broad rule of thumb in the northern region of the United States is not to plant in a greenhouse until after Valentine’s Day due to low light conditions in the winter months.

Still, experts often recommend waiting to plant early spring vegetables – like lettuce, peas, and spinach – until March and April. Warm-season veggies like these are not frost tolerant and need the added protection a greenhouse has to offer.

Because seeds take longer to germinate and plants grow more slowly when the air and soil temperatures are cool, it’s also recommended that you keep your greenhouse between 75 to 85 degrees in the day and 60 to 75 degrees in the night. Doing so keeps plants healthy and uniform.

Beyond season extension, greenhouses also provide other advantages. Growing in one ensures that crops are protected from the elements and not affected by wind, rain, or hail. And, as long as ideal growing conditions are maintained, yields are often higher for crops grown in greenhouses.

Heating a greenhouse

After you’ve gone to all the work of improving the health of your soil and planting your seeds, the last thing you want is to have a late frost wipe out all your efforts. One of the most popular ways to show the frost who’s boss is to heat your greenhouse using a propane heating system.

Propane is a clean-burning fuel chosen by farmers because it is environmentally friendly and cost effective. Growers rely on propane-powered combined heat and power (CHP) systems because they can operate at 70-80 percent efficiency while using less fuel to produce each unit of output. For you and your greenhouse, that means increased productivity and decreased environmental impact. So even when the weather in the early spring months is chilly and unpredictable, you can protect your plants against damage.

The most common types of greenhouse heating systems can be classified into two groups: central and local. A central heating system typically uses a boiler to generate heat in one spot. The warm air is then blown into the greenhouse using a forced air system. The benefit of this is that heat is evenly distributed throughout the entire structure.

Local heating systems are generally placed in the section of the greenhouse they are responsible for heating. Using radiant heat or bottom heat boilers, these systems offer more localized, concentrated heat, which in turn increases efficiency.

The type of heater you choose can depend on the crop you’re growing and the amount of heat it needs to thrive. For example, for plants that require warmer soil temperatures and higher heat, bottom heat boilers are a great option because they use more direct heat to keep the root zone warm.

Finding a heating unit

Most first graders can tell you that plants need three things to grow: heat, light, and water. Because those things are so concentrated in a greenhouse, they’re what make a greenhouse an oasis for plants. However, greenhouses can be harsh, corrosive environments for anything that’s not green and growing – things like heating equipment. That’s why you’ll want to look for aluminized or stainless steel heat exchangers, as these will help extend the life of your heater in the humid atmosphere.

Here are a few additional things to consider when looking for a propane-powered heat unit:

• A thermostat to control temperature effectively and efficiently.
• Fans to help distribute heat evenly throughout the greenhouse.
• Brackets to safely mount the heater to the greenhouse structure.
• A high efficiency model. While this may cost a little more up front, it will be more efficient and cheaper to operate over time.
• A unit that provides the right amount of heat for your operation. Heater size depends on several variables: the square footage of your structure, the type of material covering it, average wind speed, and the inside growing temperature you’re aiming for.

Propane-powered heat units are among the most popular choice for greenhouse heaters, and for good reason. Because propane is a clean-burning fuel, your propane-powered greenhouse heater will require less maintenance than oil, diesel, or kerosene models. With minimal maintenance, increased efficiency, and environmentally-friendly heat, it’s no wonder growers are turning to propane to help produce their fruits and vegetables throughout the year.

Predicting heating costs

Because no tool or person can predict the weather perfectly, it’s difficult to calculate the exact amount you’ll spend heating your greenhouse. However, the USDA offers a valuable (and free) tool called Virtual Grower 3. This software program will ask you for information such as the nearest weather station (so it can calculate typical weather conditions), type of greenhouse structure, condition of the structure, type of heating system, and price of fuel.

Not only can this tool help you figure out where heat savings can be achieved, it can also predict crop growth, assist in scheduling, make real-time predictions of energy use, and show the impact of supplemental lighting on plant growth and development. It provides a virtual laboratory where you can experiment and run scenarios to help you become a more educated, equipped grower.

Then, when you’re ready to dig in and get your hands dirty, you’ll really know what you’re doing.

For more information about greenhouse heating and frost protection, visit our Agriculture page.