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