Greenhouse Garden Tips: Starting with Cold Crops

Spinach

Schedule Your Greenhouse Garden Now

Showing off my arugula (also known as rocket) on Instagram leads to a lot of questions about my greenhouse and what I currently have growing. 

  • When can you start arugula?
  • How do you schedule crops in a greenhouse?
  • Should I start my peppers now?

Sadly, the answer is always the same: it depends. But for greenhouse growers wanting general guidelines and an excuse to get out into the greenhouse this month here are some tips from my personal experience:

Don't Delay - Start Today

Plants can't tell time! Early in the season seeds can't register warm spells or hot days. Instead, they sprout when the soil feels right and that is at different temperatures for different seeds. The earliest crops are definitely ready to plant right now.

first ture leaves

Arugula seeds are super cheap and they germinate even when the soil is cold, roughly 40 - 55 F or 4 - 12 C (Cornell University, Growing Guide – Arugula). Like many plants, arugula will germinate faster with warmer soil temperatures. So at this time of year, when I am in a bit of a hurry, I often sprout seeds in my home first.  I use a soilless mix on a heating mat. Then, I transplant them into the greenhouse once the seedlings have true leaves.

But last fall, I seeded my arugula as soon as I pulled out my tomato plants. The seed was sprinkled directly on my soil in my BC Greenhouse on October 27. It started germinating within 2-4 days in the still warm, moist greenhouse soil. Even with the days getting shorter and the nights getting colder, the arugula began actively growing by December.

Mid Feb Arugula seedsAfter heavy frost, snowy low-light days and chilly temperatures down to -8 F (- 13 C) the arugula in my unheated greenhouse kept growing bit by bit. So, by early January 2022, the seedlings, clipped with scissors, were big enough to start enjoying as microgreens in salads and on pizza. By February, longer days warmed the soil further and plants exploded with growth. Suddenly, the arugula, now over 12” tall (30 cm), is appearing in salads three days a week. When a heavy frost threatens (colder than 18 F or -8 C), I have Agribon on hand to cover young plants inside my greenhouse.

I never spray but I always rinse off the leaves once picked. After four months in the greenhouse with very little water, there is probably dust on the plants!

 

Other Cold Hardy Plant

Bok Choi, spinach, mizuna, radish, kale, members of the broccoli family and leafy greens also sprout in cool soil temperatures, around 23 F or 5 C. These hardy plants, as well as sugar snap peas, can be seeded right now (in early spring) as your soil is just warming. 

spinich

 Last year I thought I seeded outdoor pea seeds way too early on March 1, both outside and in the greenhouse. Both grew well, but my greenhouse peas were ready a full month earlier than the ones I planted outside. Seeds are cheap so I pulled the greenhouse plants just as my outdoor plants were producing.  Replacing them with small cucumber plants I had started indoors, allowed me to keep the soil and greenhouse producing non-stop.

Fewer Pests

flea beatles

The single biggest benefit with winter greenhouse growing is that the pests have gone to sleep or changed into a non-feeding state.

Flea beetles, for example, are small black beetles that hop from plant to plant outdoors in May and they often pop into the greenhouse at about the same time. They gnaw little notches into the top portion of leaves and soon enough tiny holes appear in the leaves. Outdoors in spring or summer, there is often a problem with flea beetles devastating young plants. However, in March, I have absolutely no insect problems in the greenhouse (Colorado State University – Flea Beetles).

Warming the Soil

If you can warm the soil in advance of seeding, you will get faster germination. I have used roof and gutter cables (Canadian Tire – Roof De-icing Cable) or even branded Jump Start Soil Heating Cables on the soil surface to warm the soil before I seed. Laying the cable on soil and plugging it in overnight before seeding boosts germination. Unplug and roll up the cables before the plants get over 4” tall (10 cm) or else they will get horribly tangled.

I have also used IRT mulch to warm early spring greenhouse soil. According to the University of Vermont “Infra-Red Transmitting (IRT) mulches … are pigmented to reduce the amount of visible light transmitted in order to reduce weed growth under the mulch. IRT mulches are usually green or brown, and they contain specific pigments that let them transmit a maximum of near infra-red radiation and a minimum of visible light.” See more about plastics here: https://www.uvm.edu/vtvegandberry/factsheets/plasticprime

What about Peppers?

Peppers starting tosproutThese heat-loving plants are unhappy with cold-weather seeding. They also are usually hybrids and tend to be more expensive to buy. In other words, you don't want to waste a single seed. So, at this time of year, I select the pepper varieties I want to grow. Typically, I sprinkle 6-10 seeds on a sheet of damp paper towel, put the damp towel and seeds into a plastic sandwich bag, and leave the bag on top of my grow light.  The temperature there is higher than in the room because of the waste heat given off by the fixtures.

Holding the bags up to the light daily allows you to see if the seeds are sprouting. Once you see the sprouting seeds, place one seed per soil-filled cell in a 6-slot cell pack. Sprinkle a bit of extra soil on top of each seed and water gently. Place the whole cell pack or tray on top of a heat mat or heating cables in your greenhouse. This will allow your soil to stay warm as the seeds grow.

When growing warm crops indoors, place the sprouts under grow lights as soon as the leaves poke up out of the soil. In the heated greenhouse, put the small plants on shelves or wire racks in your greenhouse. If the greenhouse is unheated just enjoy cool crops like arugula for now, and plant the peppers when day and night temperatures are well above freezing in later April or early May.

 

I can already taste the roasted poblano and spicy jalapeno peppers served with arugula on my pizza and salads this summer!

 

More from Donna

For more great tips from Donna, visit www.donnabalzer.com.

You can also read Donna’s gardening books: No Guff Vegetable Gardening with Steven Biggs and her just-released Gardener’s Gratitude Journal:  Part Diary, Part Personal Growing Guide.

DonnaLogo-01-1