Cold Frame Gardening 3: April

Jonathan weeding cold frame
Jonathan weeding cold frame

April is a busy and exciting time, especially when gardening with cold frames. While convincing signs of spring are just beginning to emerge outdoors, summer is well under way under the glass. With temperatures consistently 70° or above by day, and the danger of frost essentially gone, almost anything can be grown in the frames by mid April.

By early April, my broccoli and cauliflower plants will be pushing against the glass, exposing them to frost and sunburn. To avoid this, I have built extension boxes that essentially raise the height of my cold frames. By the middle of April, those plants are getting little benefit from the extra heat and protection of the cold frames and I carefully find the proper timing to remove the frames and let the young plants fend for themselves.

I eagerly look forward to the day when I plant out my tomatoes, peppers, and basil into the freed-up cold frames from my liberated crucifers. I again aim for the middle of a warm spell. Started in January, the tomatoes are usually bursting out of their pots, some of them with flowers blooming.

I will plant those tomatoes as deep as I can to encourage extra rooting from the stem, making sure to leave the flowers above ground for the possibility of some early fruits in May. Since determinate varieties of tomatoes have a bushing habit and do not vine like their indeterminate cousins, my cold frame plants don’t need trellising. However, the plants do have a tendency to flop over, so to prevent the fruit rot this encourages I have learned to put a hoop tunnel made from fencing for the plants to grow through, giving extra support for the luscious, pendant fruits.

This year I am growing twice as many cold frame peppers as last year. If you have ever struggled, as I have, with getting peppers to flower and fruit in the summer, cold frames may be your solution. I probably got more fruit, in one quarter the area, from my cold frame peppers than I did from the main-crop plants I started outdoors, all due to one month of extra, early protection.

The early-started plants, moreover, were spread out through the whole season and had plenty of time and fruit for early red peppers, unlike the regular plants that were mostly foliage all summer, with an excess of unripe fruits just before frost. Once again, a good example of how concentrated energy and attention can pay for itself multifold!

If you don’t have early tomatoes or peppers to plant, don’t fret. It is still not too late to start some fast-growing cucumbers and summer squash in pots to be planted out later this month. Kept in a warm, bright location, the large seeds will germinate quickly and will need to be planted out into cold frames before the roots crowd the pot, since cucurbits are not that fond of being transplanted in the first place. In fact, by mid April you could even direct seed those plants right into a free cold frame if you prefer.

Being such powerful tools of growth and transformation, cold frames do not come without certain costs and dangers. They will need constant monitoring, especially at first while getting the hang of it. Expect to learn the hard way, since all it takes is a couple of hours of midday sun without venting to find the foliage of your tender plants burnt to a crisp. Transpiration is also extreme in the greenhouse environment, so frequent watering is necessary. Opening the cold frames completely for soaking rains is ideal.

If you do find your plants burnt or withering, don’t give up. I have been amazed at how quickly well-rooted plants (much like their human counterparts) can regenerate themselves. Nevertheless, I hang onto any extra seedlings I have, in order to fill in holes caused by pests or problems. There is nothing more satisfying than a full house of plants.

It is also helpful to have family, friends, and neighbors who are supportive of your cold frame initiative. Otherwise, being tied down to cold frame maintenance will start feeling like a confining chore and not like the labor of love it was meant to be. Those early cauliflowers and tomatoes make flavorful rewards and compensation for the hard work of yourself and others.

Moreover, any mistakes and shortcomings will ripen into lessons for next year’s experiments. Already this year I have regretted my early broccoli transplant that left my tender kale and chard unsheltered. Shocked by a sudden drop in nighttime temperature, they lost most of their foliage to frost and windburn and have taken three weeks to recover. In the May article, I will share my revised timeline for next year’s sowings and plantings. Until then, happy gardening!