Growing microgreens doesn’t have to be a clean room, lab-like high cost affair. It’s very economical to grow microgreens outdoors because you don’t have the upfront costs of artificial lighting and shelving.
Microgreen farming outdoors is easy and highly scalable. It's a great way to expand your indoor farm during the summer time. That said, there are challenges unique to growing microgreens outdoors which you have to contend with.
The topics that are going be covered in this article are: Seasonality, Weather, Insects, Growing Containers, Growing Medium, and Harvesting.
Frost is a killer. But low temperatures can also hinder growth and germination.
Microgreens are vulnerable to temperature shifts. At around 10 - 15 C or 50 - 60 F daytime temperatures, there is a high risk of nighttime frost killing the plants. The damage that frost causes is irreversible. Ice crystals burst plant cell walls, killing them instantly. Moroever, the germination and growth rate is too low to be worth your time.
Moreover, un-germinated seeds would be perfect food for fungus and bacteria. Microgreens are grown in high densities, and coupled with the moisture and rain associated with spring, the risk of damping off is very high.
The perfect time to start growing outdoors is when the daytime temperature hovers around 20 - 25 C or 68 - 77 F. And with sunlight hitting the growing medium, the increased surface temperature will tremendously increase germination rates.
There’s also almost no chance of frost or insects, reducing losses. A rule of thumb for when to start growing outdoors is that if the temperature is pleasant for you to be outside without a jacket, then it's also a perfect time to start growing outdoors.
While the process of germination can take place indoors, the entire growth of microgreens should take place outdoors. The problem with starting indoors is that plants adapt to the light levels that they’re used to. It takes approximately 2 - 3 weeks before they acclimate to a higher light level. Plants that are not acclimated to daylight will burn.
Since microgreens are grown for three to four weeks maximum, the timeframe for acclimation is the same as the growth timeframe.
The sun is not your friend when growing outdoors. During the summer time, surface temperatures can reach 40 C or 95 F. Since microgreens reside near the surface, the sun can easily singe your plants.
Increased surface temperatures will also cause the growing medium to dry out, forcing you to water the growing container at least once a day.
The problem with the rain is that when it rains, it pours. Microgreens require a lot of moisture, but too much moisture also causes microgreens to be at high risk of fungal infections. Microgreens are especially vulnerable to rain drops right after germination. Size matters. Rain drops can easily disrupt the growing medium enough and wash off the seeds and seedlings.
The solution? A roof. Using 2x2 lumber or PVC piping as support structure, you can secure fabric or plastic sheathing on top of your microgreens. The fabric lets through enough sunlight for the plants to grow while providing shelter from the rain.
With the exception of bees, most insects that land on your plants are pests. While I disagree with the use of pesticides, there are organic options available.
Cabbage moths are the biggest pests of microgreens in the brassica family (kale, broccoli, radish and arugula). These white moths prefer to lay eggs under the leaves. The eggs are less than 1mm long, yellow in color and conical in shape.
In a matter of days, these eggs hatch into green caterpillars less than 2 mm long. You can inspect for insect damage by looking for holes in the leaves. Eventually, these caterpillars will grow to 3 cm long and eat your entire plant.
There are two eco-friendly method to prevent cabbage moths. Bird netting can prevent these moths from landing on the plants. However, bird netting is finicky to deal with, and not entirely effective. It can cause some stress to the moth because the moths that do get through has a hard time escaping.
The second eco-friendly method to eliminate cabbage moths is biological warfare. BTK is a bacteria that is especially deadly to caterpillars but not to humans or moths. They can be found in every nursery and have to be re-sprayed religiously every week. Make sure to wash the microgreens when you’re consuming them.
Cabbage moths caterpillars are safe to consume however. If you don’t mind the occasional caterpillar, then microgreens is an excellent source of protein.
Select growing containers that includes drainage holes and at twice as deep as your indoor containers. The increased air circulation outdoors and drainage will dry out your growing medium much faster than indoor growing.
The ideal container for outdoor growing containers are 6 to 8 inches high and rectangular in shape. The rectangular shape allows you to stack microgreens tightly. Use a drill to make holes if there are none.
For outdoor growing I prefer using plastic containers and trays. The trays act as a water reservoir when conditions are dry, while also allowing excess water to spill in the case of overwatering.
Plant roots grow at a rate of roughly 1 inch per week. To accomodate growth, your growing medium shoud be at least 3 inches deep. Older plants can tap into deeper sources of water. Growing outdoors require the container to be watered everyday for the first week, and reduced to every other day for the remaining growth periods.
Growing medium in containers dry up much faster than garden soil because it's much more exposed to the elements.
Peat based growing medium repels water, especially when its copmletely dried out. When dry, most of the water will go straight through the container and out the drainage hole. It takes at least three times the amount of water to saturate the growing medium and a lot of patience.
I recommend watering your microgreens in the morning because watering in the evening encourages fungal growth. Fungus hates light and loves moisture.
For outdoor growing, you would want a growing medium that keeps the roots are aerated and moist. This is why using garden soil for growing in containers is not recommended.
If you’re like most homeowners, you have sandy or clay garden soil. While clay soil holds water very well, it gets very compacted when wet, preventing roots from getting oxygen. And when it’s dry, it cracks like concrete, which further damages roots.
On the other hand, sandy soil is not compacted, but cannot retain any water. It’s terrible for microgreens because microgreens have shallow roots, which dies easily if the soil dries out.
You can use garden soil for microgreens if you have loam. The advantage of using garden soil is that it’s rich in minerals and microbial life. However, every garden soil has weed seeds. Even if you decide to dig several inches down, there are dormant weed seeds jsut waiting for the perfect opportunity to germinate.
So if you do decide to use garden soil, you should be able to differentiate your plants from weeds. Luckily for brassica family microgreens, their seedlings have a distinct heart shaped leaves.
For most growers, I recommend using potting soil. Potting soil is mostly made out of peat moss, which is a sterile medium that doesn’t have any weed seeds.
You can make your own cheaper and superior growing medium from peat moss: mix peat moss with vermiculite or perlite at a 10 : 1 ratio. Peat moss is usually sold oven dried and compacted. Peat moss has great water absorbing qualities, but repels water when dry. Make sure to vigorously mix with water before use.
Vermiculite is preferred to perlite for microgreens because it retains water in addition to aeration. Moreover, you can add fertilizer to the growing medium and/or rock dust to enrich the mineral content.
If you’re new to fertilizers and don't know how much to add, all fertilizer products have 3 numbers written on the bag such as 10-10-10. This means that it’s 10% by weight Nitrogen – Phosphorus – Potassium respectively. The final concentration in the growing medium should be at 0.5%. To get the amount of fertilizer you should add, measure the weight of the growing medium, then divide by the first number and then divide by two. Try not to mix too much fertilizer because it will burn your plant.
The seeding rate for growing outdoors is the same as growing indoors. Germinating indoors or in the shade is preferrable than directly sowing at the growth site. It's easier to maintain higher moisture and humidity indoors.
Growing outside is better than indoors because the microgreens get a better quality light with sunlight rather than artificial light. Microgreens grown outside should grow faster than the ones grown indoors because outdoor lighting is at least 5 times brighter than indoor solutions .
Harvesting is just as easy since you’re growing them in containers, with a sharp pair of scissors
Make sure to remove the roots if you are re-using the growing medium. Spent peat moss is also a great addition to a compost pile due to its carbon content, and soil amendment for lawns due to its humic acid content.
I am the author of this website and owner of growyourmicrogreens.com. I am an hobbyist gardener and a passionate scientist. I was trained as a scientist in the Molecular Genetics program in University of Toronto, where I received my Masters of Science and published a journal article.