Monthly Archives: May 2013

Physics on the Farm: Brassica in a Whole New Light

At the farm, we gently wash the vegetables in preparation for the distribution. It’s a meditative process: gently we lay the earth bedecked root crops in the first tub of water. Swish, swish! Swish, swish! One can imagine radish tops as the tail of some exotic koi. One by one, each vegetable in turn, passes through a couple of changes of cool water, so that they’re free of clods and are radiant when you pick them up.


One afternoon, while washing the collard greens, John noticed that the leaves took on a silvery sheen when submerged. Green above water, silver below. What was going on? The answer is a combination of botany and physics.

Collard Green leaves (as well as the leaves of other Brassica) are covered with a waxy cuticle, a waxy layer that the plant secretes to deter pests from munching its leaves. The waxy cuticle makes the leaf slightly waterproof and that means air bubbles adhere to the surface when the leaf is plunged under water. (Fire ants take advantage of a similar development in their exoskeletons when they make waterproof rafts of themselves to cross rivers or survive floods … but that’s another story!)

But why would a miniscule layer of air look all silvery? This is where the physics comes in.
Light bends when it travels from one medium to another medium of a different density. In the case of our submerged collard green, from the water into the air bubble on the leaf’s surface. When passing from a more dense (water) to a less dense (air) medium, it is possible for the light to get “trapped” in the bubble and not be refracted back out again. This happens if the angle at which the light enters the less dense medium is greater than 48.6 degrees. At that angle, the light entering the air bubble is reflected off the boundary between the air and the water and does not refract – bend or have it’s speed changed enough to pass back through the boundary. This results in what is called ‘total internal reflection’, and we see a silvery surface. Neat, huh?


For a more detailed explanation of the physics involved see:

For more on the fascinating fire ant rafts see:


Why Has Spring Been So Cold?

 “The first day of spring is one thing, and the first spring day is another.  The difference between them is sometimes as great as a month.”

Henry van Dyke in Fisherman’s Luck (1899)

             What lovely weather we’ve been having!   Cool evenings, bright, pleasant days, with a bit of rain now and then. True, it’s not the best for growing – a little warmer and wetter would be a little better – but this year’s cool is far better than last year’s brutal heat wave and drought like conditions.  March, the first month of spring, was more than 4°F colder than this past December, the first month of winter.  The March 2013 average temperature was 11.3° F colder than last March’s.  But why has it been so cool?   It has much to do with something called the Arctic Oscillation.

The Arctic Oscillation is a combined measure of barometric pressure and wind current direction and speed that describes the state of atmospheric circulation over the Arctic.  A “positive” Arctic Oscillation means the occurrence of lower pressures and swifter wind speeds preventing the seepage of cold air into the south which results in warmer than average weather with less snow for the eastern U.S. (See the orange bars in the graph below.)


Image from:

           The “negative phase” occurs when there is high pressure in the arctic, and the ring of winds circling the pole are weakened allowing the colder arctic air to plunge into the lower latitudes … our latitudes!  This means more late spring snow as recently experienced by the Midwest, and cooler, drier conditions for us.  But it’s warming up, and the rain is on its way.  In fact, the warmth is good news for our tomatoes and eggplants which, as you can see, are nearly bursting from the greenhouse in their eagerness to get their roots into the soil and their leaves into the sun.


Hooray for Spring!

What can you do with so many radishes anyway?

A unique shot of our radish seedlings a few weeks back.

Our radish seedlings a few weeks back.

With the cool spring we’ve been having many of our crops are growing slowly, but our radishes are thriving!

This unique abundance sent us searching the web for recipes–what do you do with so many radishes anyway?  Here are links to what we found (and an idea of our own).

Radishes with Butter and Sea Salt

Roasted Radishes with Radish Greens

Buttered Radishes with a Poached Egg

Deviled Eggs with Radishes

For a simple and surprisingly lovely side dish, slice radishes thinly, sauté in melted butter.  Cook them to your taste, about 5-10 minutes.