The sharp blade slices through the skin, flesh and vascular tissues with ease.    Pressure in the vascular system collapses.   Almost immediately senescence – a genetically regulated process which leads to the death of cells and organs – begins.  Individual cells react by discharging destructive enzymes:  Polyphenoloxidase, which starts a process of enzymic browning; Lipoxygenase, which promotes oxidation of the organic tissues; Lipase, which sets into motion the process of lipolytic rancidity; and Protease, which starts to soften the texture of the flesh and skin.    The metabolic properties of the organism are fundamentally disrupted:  Proteins, water-soluble vitamins, antioxidants and nucleic acids, instead of being generated as they are in living things, start to degrade.  At the same time, the rapidly dying appendage also starts to lose its ability to control its microbial population:  bacteria, mold and fungi begin to multiply, also consuming the nutrients that had been available to the cells.  Toxic and infectious bacteria start to reproduce unchecked.   

A macabre pre-Halloween account of a severed limb?  No.  I have just described what happens the moment a leaf of spinach is harvested from the host plant.  And 48 – 96 hours later (depending on the temperature), half of that spinach leaf’s folates, carotenoids, vitamins and other nutrients are lost.   So if your local farmer harvests the spinach on Thursday or Friday, sells it to you at the farm market on Saturday, and you eat it in your salad on Sunday, it is probably half as beneficial to you compared with the spinach leaf plucked from your roof shortly before tossing the salad on Sunday.    

Although not widely used, “half-life” is a concept that attempts to quantify the perishability of fruits and vegetables.   Although individual nutrients, like vitamins, enzymes and proteins, degrade at different speeds, the “half-life” measure looks at all the food components generally viewed as beneficial to humans, and estimates the length of time following harvest it takes for these nutrients to be reduced to 50% of the levels present in the plant when it was alive. 

 The notion of “half-life” ought to be a central part of the case for urban agriculture, and should inform our choices of what to grow at home.   My roof-top orchard, for example, contains both peaches and apples.   Looked at through the lens of “half-life,” my choice of peaches is a good one, because any peach that has travelled (or ripened off the tree) is a sorry shadow of  the same fruit that is tree-ripened and twisted from the limb moments before entering your mouth.  My choice of apples, on the other hand, is revealed to be an indulgence that is insupportable.  I gain little from the effort to coax a few apples into being on the 35^th floor, and should be supporting Hudson Valley farmers, whose apples are undiminished when they reach the markets in Manhattan and, indeed, may well be superior to their urban counterparts due to the superb terroir for apples in up-state New York.

Of course there are many reasons other than peak nutrition to grow food in the city.  But for the rooftop farmer who wants to give priority to those plants where the benefits of immediate consumption are greatest, what are the best choices?  The internet is awash in contradictory information on the perishability of fruits and vegetables, much of it from studies sponsored by companies whose businesses involve the freezing, canning, transportation and/or marketing of food.   So please don’t afford the following the authority of independent peer-reviewed science, but take it for what it is, my own compilation and summary of those studies which seem to me to have the most credibility:

Shortest “half-life”:  lettuce, spinach, kale, endive, peaches, parsley, broccoli, asparagus, sweet corn, green beans.

Average “half-life”:  squash, eggplant, peppers, cauliflower, apricots.

Longest “half-life”:  apples, winter squash, oranges, cabbage, carrots, lemons, beets.

Dead or alive?  One of the great joys of rooftop agriculture is the chance to eat your plants alive.   Once you do, it’s hard not to look at the shiny products in the supermarket produce aisle as vegetarian road-kill.