Why build a cold frame for growing vegetables and the supplies you will need


In the southeast corner of our backyard, we initially had a vegetable garden which was 15’x15′ at ground level.  This garden was sufficient for the first couple of years we lived here because there was only a 20’ maple tree to one side of the garden and some 5’ blue spruce trees towards the back of the yard.  But, as our interests in adding more edible landscaping to our yard grew, so did the number of plants growing in this particular area of our yard.  We added a tall lattice in the corner and planted three grape vines, and we also added three gooseberry plants, raspberries and some rhubarb.  All these plants were now competing for soil nutrients and water.  In the spring, we would go to turn over the garden and find the maple roots were hard to work around.

Here is where the idea of building some raised gardens in the yard made sense.  I knew that, not only did I want to make it easier to work in the spring, I also wanted to increase the number of vegetables I could plant and also add some interesting focal points to my yard.

At this point in my gardening life, I had owned three houses and created different vegetable gardens at each of these locations.  I had read several books describing how to square foot garden and so set out to design a cold frame that would be efficient for growing the vegetables that I cherished most.  These included, radishes, onions, lettuces, spinach, kale, beets, carrots and leeks.

In planning my edible landscape, I knew I wanted garden boxes that would allow me to plant my vegetables as early as possible and also allow me keep the plants going right up until the winter.

I decided we would start with one cold frame with a hinged, but removable, Plexiglas lid.  Once I knew how many vegetables I could get out of the first cold frame, I could add one or two boxes each year.  As of March 2020, I have two with lids and two without lids in my backyard.

Now that I have given you the background on why and how we made the decision to get into raised beds, here is what I would suggest you do.  It is ideal for plants to have soil that is as alive as possible, meaning good microbes, worms etc. So, with this in mind, build your cold frame at least one month before planting in it.  If you have an existing garden and can take some live soil out of it to partially fill your cold frame that is ideal.

Here is what you will need to build your own cold frame:

  • Lumber. The amount and dimensions you need will depend on how large the cold frame will be.  I just use standard spruce lumber – easily available at any building center.  You want to build the cold frame large enough that it won’t dry out quickly during the hottest days of summer.  I would suggest that, at a minimum, you make it tall enough to allow for the soil to be eight inches deep; if you are going to plant large plants (like tomatoes) you will probably want 12 inches of soil.  As for the width, I like 36 inches.  This provides space for six lettuce plants across, and yet it is not awkward to reach all parts of the surface.  The length is more of an aesthetic choice.  So, for example, with my first cold frame, I made it 24.5 inches high, 36 inches wide, and 72 inches long.  I wanted the top to be sloped when the cover was down.  So, while it was 24.5 inches at one end, it was only 20.5 inches at the other.  You may be thinking that 24.5 seems like an odd choice – why not 24 inches.  Well, standard lumber has a half inch planed off to make it smoother.  So, I used two 2x10s and one 2×6 stacked on top of each other to build the sides.  But, a 2×10 is actually 1.5×9.5 and the 2×6 is actually 1.5×5.5, which means the sides were 9.5 + 9.5 + 5.5 or 24.5 inches.
  • Paint. Use a latex exterior and put on at least two coats.  I don’t use treated lumber for the cold frame, as I don’t want any chemicals leeching into the soil.  So, the paint is needed to protect the wood from insects, water, and sunlight.  The paint is also an opportunity to add a splash of colour, which you might really appreciate in the winter.  Once the wood is cut to the appropriate dimensions, I paint it before assembly.  In that way, I ensure that all parts of the wood are covered by paint.
  • Landscape fabric. Once the cold frame is assembled and in place, I lay landscape fabric on the bottom.  I use enough so that it curls up and covers an inch or two of the sides.  This will help hold the soil in when the cold frame is new.  Eventually, the cold frame will settle firmly into the ground, but when you first build it, there may be some gaps around the bottom.  Landscape fabric will allow water to pass through.
  • Clear plastic sheeting. You can find this by the roll at the building center.  I use this to line the inside of the cold fame.  It keeps the wet soil from directly contacting the wood sides of the cold frame.  You will want to leave a gap between the top of the soil and the top of the cold frame.  So, only line the sides as high as you need to.  For example, in my first cold frame, I wanted the soil to be about 12 inches deep.  So, I only lined the sides up to the 12-inch mark.  In that way, you will only see the painted wood above the soil line.
  • Soil. This is where you can apply your own recipe.  One thing I would suggest, however, is that you include some soil from another garden in your yard.  If all you use is bagged soil your garden will lack important soil microbes.  So, I always like to add some living soil.  I also like to include compost and manure.

Keep in mind if you are planning on building your cold frame in April and then using it in May you just have to build the box part and do not have to worry about building the lid until the fall or next spring.  Better to get the soil into the box and let it settle for a month so you can get to the planting.

Why do you need to wait a month before planting?  If you have added compost to the box, and you should, you should allow the microbes a chance to break down the compost, in particular the nitrogen.  If you plant seeds directly into the bed after adding all the soil the process of the microbes breaking down the compost might interfere with germination.


Peppers – worth considering for your garden

Home grown habanero peppers
Patio pot grown habanero peppers

Peppers are such a versatile vegetable to have on hand in your pantry.  They are great fresh, amazing in a stir fry, a chili or soup, and they maintain a lot (80%) of their nutrients when canned or frozen.

Peppers keep well in the fridge – if unwashed, up to 10 days.  They also do not  require blanching before freezing, making it convenient to save for use in the winter months.

Green bell peppers 2018“Bell peppers” often just referred to as red, orange, yellow or green peppers are very nutritious.  One red bell pepper has twice the amount of vitamin C as an orange! These nutrient packed vegetables also have vitamins A, B6, K, E and elements such as potassium, magnesium, phosphorus and manganese.  Bell peppers are also full of antioxidants, making them a tasty super food.


Peppers are rated for heat in Scoville heat units with bell peppers at zero.  Jalapeno peppers have around 3500-8000 units and, well one of my favorite peppers habaneros, pack between 150 000 to 300 000 units.  There are even hotter peppers, such as ghost peppers. So, if habaneros do not pack enough heat for you, grow or pick up some ghost peppers at your local farmers market.

If you would like an easy way to add “kick” to your recipes, hot peppers, such as habaneros, dehydrate well using a dehydrator.  Just wash, quarter and seed your peppers, and then process them according to your dehydrator’s instructions. Then store them in a sealed container.  When you are ready to use them, blend them in a coffee grinder or food processor. I don’t want to breathe in the powder, so I blend my dried hot peppers outside or in the garage.

You may have heard that green peppers are just under ripe red peppers.  While it is true that some green peppers will in time ripen to red. Some varieties of green peppers will stay green throughout their maturation process.  I love buying my peppers for salsa making at the farmers markets because I can get a half of a bushel at a time. I like to see some peppers, in a bushel of bell peppers, that have started to turn red because that means if I want to use them as a red pepper, I just have to leave the pepper at room temperature for a couple of days, and it will turn red.

Is the red pepper more nutritious than a green pepper?  I have read that a green pepper will have less vitamin C and A compared to a red or orange pepper.  However, if you prefer the bitter taste of a green pepper, eat it!  Consider it to be part of “eating your greens”.

Fun fact, peppers belong to the nightshade family of plants.  Other nightshade plants include: chili pepper, cayenne pepper, eggplants, tomatoes and potatoes (but not sweet potatoes or yams).  It is rare, but some people have severe digestive problems when eating night shade plants so if you are finding that all the above fruits and vegetables give you some issues, you might want to avoid the nightshade family of plants.

Regarding planting peppers

jalapenos 2019
Jalapenos grown in a cold frame 2019

It took me several years to figure out how to achieve success in growing peppers in my home garden.  When I first started home gardening in Southern Ontario and was just starting to get into growing vegetables, I would buy a couple of greenhouse jalapeno pepper plants each year.  I would basically plant them and water when needed and I would be lucky to see a couple of peppers each year from each plant. It hardly seemed worth the effort.



But I am stubborn, and so over the years I have tried different pots and locations with a variety of sun and wind exposures and have found that this has resulted in the best success in growing peppers:

    1. If starting from seed indoors make sure to start your plants early.  For instance, for poblanos (ancho) peppers they should be started 5-6 weeks before the last frost. So, where I am these can be seeded indoors as early as mid March.  Habaneros can be seeded indoors even earlier.
    2. If you are going to purchase plants from a green house, choose the straightest, healthiest plants and buy your plants early.  Even if you have to baby them a bit at home before they are ready to be transplanted, by buying early you will have a good selection of healthy plants to choose from.

    3. Choose a very sunny location, at least 8 hours of direct sunlight

    4. The most successful vessel for growing peppers has been my cold frame.  We amend the soil with compost and manure.

    5. My cold frame is in a nice location, no wind issues and full sun.

    6. Seed package instructions will tell you to make sure your soil has an abundance of phosphorus and calcium. So to do this, you can add lime when applying the compost to the bed or container before transplanting.  I have found that putting both compost and manure in my cold frames has been sufficient.

    7. And most important, prepare to fertilize your peppers.  I scratch in an organic pellet fertilizer after transplanting the plant and then scratch in additional organic fertilizer every 3 weeks.

    8. Peppers will tolerate dry soil but you will get good plant growth if the soil is kept moist.

    9. Be patient, peppers do take a long time to develop.  If you don’t get your transplants in until the end of May or June, you might not have peppers until August or September.  But again, be patient, because some of my best pepper plants produced the most peppers ready for harvest at the beginning of September.

      Poblano peppers grown in a Mayne planter with reservoir

Lettuce and kale are not the only plants that you can start in March indoors from seed

seedlings in cold frame
Seedlings planted in the cold frame

Yesterday, I posted a video about how to start lettuce and kale plants from seeds. Once you have planted the lettuce and kale seeds, place them in a warm place in your house and water the soil when it is dry.  The seedlings should sprout in just a couple of days.  If you have a south facing window, put your seedlings near that window during the day so that the plants can get some filtered sun.

Before the plants can be put in the garden, they must be hardened off.  This is simply the process of acclimatizing the seedlings to the outdoor conditions.  Having been inside, they will sunburn easily and they are not strong enough to withstand the wind.  So, start by putting the plants outside for an hour out of direct sunlight and sheltered from the wind.  Then gradually expose them to more sunlight and wind.  For example, you could put them out in the morning sunlight, which is less intense for an hour one day, followed by 2 hours the next day.  Once they tolerate morning sun, allow an hour of midday sun and increase that over the coming days.  If the seedlings are getting too much intense sunlight, you’ll see a bleaching of the leaves, which can turn brown if they get burnt.  Similarly, you will want to acclimatize the seedlings gradually to the cooler evening temperatures.  They should be ready to transplant after a week.

Then, once the weather is around zero degrees at night, I plant lettuce and kale seedlings in my cold frame which has a Plexiglas lid that I can close at night to trap the heat.  When the weather is 10 degrees or more during the day, I prop the lid open to allow some air flow.

If you do not have a cold frame and want to plant them in a container or in your garden, then first, make sure your seedling has 3-4 true leaves and for best success plant after the last frost in the spring. 

Additional plants to consider starting from seeds indoors in March are:

  • Arugula, broccoli, cauliflower, cabbage, onion, peppers, tomatoes, eggplant and sweet potato.
  • Parsley can be started as well.

Because tomatoes and peppers need a long growing season, I start them indoors about 6 weeks before I plant them in the garden.  So, I’ll use a larger container – 4-inch pot, for example.  But the planting method is basically the same as kale or lettuce.  To increase my odds of success, I plant two seeds in each pot and simply trim one with scissors, if both germinate.

I usually start celery in March indoors because it takes 20-30 days for the seeds to germinate and so you might need 10-12 weeks before they are ready to be transplanted.  Transplant celery carefully, it is a little more delicate than the other plants listed above.

I have planted arugula from seedlings and also have directly sown the seeds in my garden.  The best success I have had was from transplanting seedlings and so that is usually how I plant this vegetable.  If you haven’t tried arugula fresh from the garden, you must!  I find that the fresh picked arugula has a more pronounced peppery flavour.

Broccoli transplants extremely well. I have had great success with broccoli and would definitely recommend starting some plants from seed.  The plants themselves can take at least a square foot in your garden so make sure you plant wisely and only seed the number of plants your garden can support.

I generally do not plant cauliflower in my garden, as it takes a long time for the head to form and I have had problems with pests growing this vegetable; so this is one that I pick up at the farmers markets in the summer time.  Hey, if you have had success with this vegetable in your home garden, great!  I would love to hear some advice on growing this vegetable.

I love growing cabbage plants and Brussels sprouts!  They are fairly easy to grow and look fabulous in the garden, especially Brussels sprouts.  I have had some years where my Brussels sprouts have turned out great and some years where the plants have done very poorly.  I still recommend trying them because they are just beautiful, almost stately plants in your garden.

Onions are fabulous to have in a home garden! It is so nice just to pop outside and pick a fresh onion for a salad or a meal you are about to prepare.  I like to cheat a little bit and plant the onion sets at the end of March which usually you let grow to a mature palm size onion. However, I pick some early and just use them as you would green onions.  Alternatively, you can plant onion seeds and grow them into seedlings indoors and then transplant them once there is no longer a risk of a frost.

I have not yet tried to grow eggplants from seed but have had great success growing eggplants from nursery garden plants.  You do need a good amount of summer to grow this fruit.

Last year, I picked up sweet potato vines from a nursery and was extremely pleased with the results of growing this vine.  The potatoes were small but very yummy!

In March, I can’t hardly wait for fresh parsley from the garden. So, I do like to get a couple of these herbs started, as well. I grow Italian parsley and find this herb super easy to grow.  Seed many plants, you will not be disappointed! 

Lettuce after a few weeks
Lettuce after a few weeks