The waiting is the hardest part: Hardening off seedlings

I have three weeks to go until our average last frost date.

coldframe full

I did really well this winter, getting seeds sown and potting things on. Now, I’ve got to hang on until the tender things are safe to plant. The cold frame is jammed full.

Hardening off seedlings, though, is too important a process to skip. I skipped it all the time until I built my cold frame–mostly because I didn’t want to spend half the morning bringing trays of plants in and out of the house–and my plants performed correspondingly poorly.

Hardening off is a process of gradually exposing seedlings and tender plants to larger temperature variations and stronger light levels than they experience indoors. The process produces slower, sturdier, more resilient growth, preparing plants for life in the open garden. Without a cold frame, gardeners place plants in a sheltered location outside–maybe in a shaded site, close to the warmth of the house–for a short period of time, lengthening the exposure to outside conditions every few days.

Using a cold frame–basically a tiny greenhouse–makes the process far less tedious. It’s not quite a matter of set-it-and-forget-it, but it frees up lots of the time and space other parts of life demand. My cold frame, which I constructed myself from recycled materials for about $30, faces south and backs up to my house. I start hardening off my seedlings in mid-February, when we begin to see a few 50-degree (10C) days. I start with the hardy perennials, then move in the hardy annuals. Tender plants come last. Space permitting, I sow vegetable seeds in flats directly in the cold frame.

During the first few days in the frame, I place plants on its south side so they sit in slight shadow from the frame’s front wall. They gradually move towards the back as more plants come in. On unusually warm days, I vent the cold frame by propping open the glass doors, made of discarded windows. This keeps the temperatures inside from getting too hot–such fragile plants can quickly dehydrate and die if the temperatures climb too high. Sometimes I’ll prop open the windows entirely, but cover the opening with a bit of horticultural fabric. This practice lets the light in but keeps hungry foraging birds and squirrels and early insect pests out.

coldframe full 2

The second quadrant of the cold frame.

We’re getting close enough to the last-frost date that I may move out some of the plants with the longest tenancy–particularly the perennials–into permanent locations. They’ve withstood some cold nights already, so a late frost won’t likely bother them.

Believe, gardeners! The digging season is nearly here.





Garden log, 10.27.14

Just a quick reminder to myself–I transplanted broccoli raab and spinach today, watered the plants in the cold frame and the potted cyclamen. It’s been dry for about ten days now. The nights have been nice and cool but today got up to 80 F (26.6 C), which doesn’t feel like fall at all. We’re also late for our first frost, but it doesn’t look like we’ll have any chance of it before Saturday at the earliest.

I’m starting to get enough falling leaves to be worth raking and shredding. My personal promise to myself is not to let the leaves get too far ahead of me. It’s depressing to have to spend an entire weekend raking and shredding (but oh, the wonderful leaf mold!).


Garden log, 1.17.14


I went outside to vent the cold frame this morning, and discovered that a villainous squirrel found his way into the cold frame yesterday and dug up and discarded all but two of the lettuce seedlings I had planted in a new salad bowl garden, thus setting my salad consumption back by about eight weeks. Must dig out the bird netting a bit earlier than usual, I see.

Cold frames

Welcome back, gardening friends! I hope you had a wonderful holiday season and a happy new year.

The deep freeze that has hit most of the eastern half of the United States has just found its way to me. Fortunately, over the long holiday break, I took advantage of some warmish weather to repair my cold frame, which took a bit of a beating during the recent home renovation.

A cold frame is a simple enclosure with a clear or translucent roof that is used to shelter plants from cold or otherwise inclement weather. It is similar to a greenhouse, only generally less grand, and specifically, unheated. It can sit low to the ground or even be dug into the ground. It can shelter pots or can be planted directly, depending on the user’s needs.

I built mine out of recycled materials three years ago. It is a simple wooden box, measuring 8 feet long by 3 feet wide, and slopes from 20 inches tall at the back to 15 inches in the front.cold frame

Because the area where it is located contains the main electricity line to the house, the box sits on the surface, atop of a thick layer of gravel. In a colder climate, I might dig the cold frame into the ground, but it seldom stays very cold here for long.

The frame windows are upcycled window sashes from the Habitat for Humanity ReStore.

pots of greens in the cold frameIn the past, I lined the perimeter of the frames with foam weatherstripping, which does a good job of keeping out the cold air. However, it becomes brittle after a season and must be replaced annually.

The white-painted interior reflects light to the plants inside. The cold frame faces south, but tall trees to the southeast block some sun, even in the winter. (I’ll paint the outside to match the house when the weather warms up.) The cold frame does an excellent job of sheltering  tender plants, like my salad bowl garden. It also keeps my worm composting bin frost-free, allowing the worms to continue their good work, albeit more slowly, in the coldest months.

broken window panes on my cold frame

One of the window sashes was damaged during the renovation and must be replaced. I have been slow to fix it because its sorry state has spared me from having to vent the box during the warmest part of the day. I cannot replace the window today, but with overnight temperatures expected in the single digits Fahrenheit (that’s very cold for here), I need to make some arrangements to compensate for the heat loss the broken window allows.

First, I’ve crowded my plants together tightly in the section furthest away from the broken window. Reducing the gaps where frigid air can circulate will help the plants survive. I’m moving a few of my winter sowing seed pans and cuttings into the frame as extra insurance, though that does go against the philosophy of winter sowing.

crowded pots in the cold frame

Next, I took a few old shopping bags from my shed, filled them with dry leaves, and tied the handles closed. I tucked these in amongst the plants inside the frame. These will act as makeshift insulation batts, sheltering the plants from any drafts.

Third, I cut a double-thickness section of 3.5-mil translucent plastic sheeting to fit the window frame and stapled it into place, to help compensate for the broken panes of glass.

A double thickness of 3.5 mil plastic sheeting compensates for the broken glass panes.

Finally, below the window sashes but above the plants, I added a layer of that same 3.5 mil plastic sheeting and some cardboard before it goes to the recycling. This should stop most of the cold from penetrating the broken window.

cold frame put to bed

I know my northern neighbors are coping with much worse weather, and my thoughts are with you all. Stay warm and safe!