Gardening at the Boundary

That day, maybe a week ago, it really came down.

I know nothing about late spring snow. Nothing. When I was a child, in New York, it snowed during our Easter break -it was early April. The day prior was warm, even the day it snowed it was warm, so much so that I was out riding my bike in the street with my brother. Although it was cloudy, the big, wet flake snow came without warning.

This snowfall is different, intermittent pellets and flakes. It was windy too, driving the pellets hard. As is often the case, the snow did not stick. The snow was not the trouble at all. It was the cold that presented itself the following night. 

I woke to find a frost on the little wetland.

 Crystals coated all the leafed out, saturated-looking plants in the early sun.

The parsley I had just planted showed crystallization along its veins (interesting that this happens, no?).

The  cilantro.

The Virginia Wetleaf succumbed (but recovered) to the eight or so hours well below freezing.

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The last frost date for our location is roughly May twenty. I do not think anyone would suggest that the last two months have had ordinary temperatures, we haven't. Since March, we have had days that topped out at 10ºF and 82ºF, although most have been in the forties through the sixties. Our March monthly average high temperature was nearly 46ºF and the April average so far is 59ºF. Daytime temperatures have long suggested I should be growing things that California is having trouble providing. Think twice. I watch the trees and the vegetable gardens. Only this week are the oaks beginning to show the chartreuse of spring and there has been zero garden activity.

Warm air masses, heated by their descent from the Rockies and Great Plains, move in from the south and west, and locally there is sunlight warming the thermal mass of land without the cooling influence of great bodies of water. The day warms nicely. At night, however, without the moderating influence of clouds, radiational cooling is strong. I recall a typical temperature differential in NYC to be about 15 degrees. Here, in Minnesota, I have seen 20+ as the norm. Beyond nightly cooling, there is always the threat of a cold airmass coming down from the north whenever the jet stream decides to do something funky. Minnesota is the common entry point for cold air, it is the reason people think this state is cold. 

Which brings me to another weather detail. I noticed the window box of just planted pansies was bone dry. What? I had watered it in, deeply, just the day before. Hmm. Something unusual had happened -dry air, exceptionally dry air. Two days after the snowfall, and the day of the overnight freeze, our relative humidity had dropped to 12%, twelve percent! Our dewpoint was nearly 1ºF by the late afternoon. Meanwhile, our high temperature was 55ºF and the winds were up. The water simply evaporated. Despite this, the pansies toughed out the freeze and drought, as those in the pot above attest.

The dry air, the sudden cold from the north, the high temperatures, the wind, no rain, and of course, heavy rain are all typical. We live at a climactic boundary with little to moderate each influence. This is the education of a gardener.

New York State Frost Dates

If you are wondering "when is that last or first frost date here in NYC," below are two maps,  courtesy of the wonderful people at Cornell University, of first and last frost dates of the New York State season. These frost dates are "roundabout," so it is always wise to follow the weather when thinking of planting tender plants or deciding whether or not to harvest those last few tomatoes. Any given year we can push or pull these frost dates. Looking at those dates tells me we in NYC are very lucky indeed.

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How I Made My Cold Frame

Below are pictures of the cold frame I made. I have a table saw at work, so this made the job easier than if I had to make it at home. But cold frames can be made from a variety of things, like stacked bricks or cement blocks with an old glass window on top. If you are making it out of wood, you could just jigsaw (or even hand saw) some plywood into a similar pattern and throw a piece of plexiglass (or even plastic sheeting) on top.

The pattern can be as simple as a rectilinear box, but I sloped mine so I could let more sun into the box and allow rain or snow-melt to run off the lid. Yours could be set onto or into the ground. I will raise mine a few inches with some screw-on wooden legs because I do not want to smash any underlying plants (upcoming bulbs, particularly).

My cold-frame has quite a small footprint at roughly 28 x 18 inches, but they can be much larger and taller. I designed mine for a city gardener, someone with a small garden and not too many plants to start.

I used scrap wood left from student projects at work. This wood is primed, finger-jointed 5/4 pine. It is 1 and 1/16-inch deep by 5 and 1/4-inch wide. In order to get the height that I wanted, I used a dado to rabbet-joint and glue two pieces together, making for a 10-inch wide board. This gave me a maximum height of 10 inches for the rear plank. The side planks are cut on a diagonal, sloping from 10 inches down to 8 in height. I bevel-cut the top of the rear plank to accommodate the slope of the side planks.

I cut rabbet joints at the four corners to more securely hold the four sides and to help keep out cold air.

I cut the front plank at 4 inches high, leaving another 4 inches for a piece of polycarbonate glazing. I dado cut a groove into the top of this front piece to snugly hold the glazing. After that, I bevel-cut the top of the front piece at 30 degrees to help shed any water.

This view points to the inside-bottom of the cold-frame. I rabbet-cut the bottom to accommodate planking that will be the floor. Exterior water will shed without contacting the interior floor planks with this set-up. However, if you set yours on or into the ground, floor planking isn't necessary.

These views show the polycarbonate glazing on the front. I used glazing here to increase the amount of light reaching the plants inside. You can see how it is held tightly within the groove on the front plank. I cut the glazing 1/4 -inch taller between the side planks so that the roof-glazing would make contact with it.

The roof glazing is a sheet of double-walled polycarbonate set into a dado-cut groove in the wooden frame. The rear of the glazing-frame can be seen below resting on the back planking. Out-door hinges will attach the roof glazing frame to the cold-frame.

This is the cold-frame with the roof glazing on. I left the plastic film on the glazing so that I know which side goes out.

This is a close-up of the roof and front panel glazing. The glazing is held snug in the dado-cut grooves in the wooden frame. The roof glazing overshoots the front plank by 1/2 -inch so that rain drips beyond the frame.

For now, I will use a stick to prop open the cold-frame for venting.

The joints will all be set with waterproof wood glue and out-door quality screws. I will paint the cold-frame to protect it from weather and sun damage. If I had made this out of cedar or redwood I would not bother, but this finger-jointed pine is really meant for interior applications. But with a good couple of coats of paint, it should last long enough. I have some old black barbecue paint that I think will do for the outside. The inside I'll paint with glossy white house paint. The idea is to not spend any money, or more than I have to. The polycarbonate cost 30 dollars at Canal Plastics, and that's about what this whole project is worth to me.

I see that I could buy a really nice one at Johnny's for $325 plus shipping. Maybe in better times. I could also add an automatic roof opener (I actually have a couple of these, but they're in Minnesota, I think). These openers are often wax-filled cylinders. The wax expands as it heats up and pushes a bar which opens your roof. The roof needs to be lightweight for this and the polycarbonate fits the bill.

Tom Chrisptopher at Green Perspectives has some good points on the use of a cold-frame. The kind he describes is much larger, and I like his idea of using the removable-pin hinges as a way of connecting the side planks. His point about "managing" the opening and closing of the roof is well taken. I want to experiment to see how it goes, but will get the auto-open cylinder if it becomes too much hassle.

Spring and Fall Frost Date

If you were wondering "when is that last frost date here in NYC," below are two maps of first and last frost dates of the New York State season, courtesy of the wonderful people at Cornell University. These frost dates are "roundabout," so it is always wise to follow the weather when thinking of planting tender plants or deciding whether or not to harvest those last few tomatoes. Any given year we can push or pull these frost dates. But looking at those NYC dates tells me we in the city are very lucky indeed.

Click on the map for a larger view