It seems like our switch over to a new newsletter email delivery scheme was fairly uneventful. Hopefully everyone who wants to receive the newsletter via email is receiving it via email and those who don’t want it have figured out how to unsubscribe. If you are not receiving this by email and would like to please subscribe here. If you are receiving this by email and don’t want to please click the “Unsubscribe” link at the end of the email. If anyone has any questions feel free to send me an email!
If you are interested in adding fruit, eggs, coffee, flowers, jam, or mushrooms, please do so soon. Though we can prorate the price if you start in the middle of the season it is far better to get the full share worth! You can order them online.
Last year we had special on-farm pick-up hours for members who were at high risk concerning COVID. We can continue to provide this service if there is interest. Please let me know ASAP if you would like us to continue this service. If we do continue it the pick-up time would be 9:00 to 9:30 am on Wednesdays, same as last year.
We are not requiring people to wear masks in the pick-up tent this season. If the state changes their guidelines we will reexamine this approach. Unless or until then feel free to be mask-free (or wear one, either way is fine).
Farm News
I hate always talking about the weather but it is such a huge factor in our business that there is little else of substance to discuss. So lets get right to it. Pay close attention. This will be on the final.
At the end of last week we asked for volunteers to come snuggle with our frost sensitive plants due to the potential late frost forecasted for the area. No one said they were coming but I suppose there could have been a few of you who snuck on to the farm and helped save a few of our plants. Thank you whoever you are. Well we did have two nights of frost last week, the last night being the latest we have ever experienced a spring frost. The most adversly affected crop was the cucumbers where we lost approximately 30%. Don’t worry though, with this being only the first planting of cukes we will have plenty of cucumbers over the course of the season. In addition, some of the early corn looked a little frost damaged but no significant loss. Some of our squash plants lost a couple of leaves but all, for the most part, survived. The tomatoes, peppers and eggplants look fine. The okra took a bit of a hit but again not a significant loss. So all-in-all it was just a minor negative event.
However, to put it in perspective, we have a member of the farm who rents some land from us for his own personal farm project. He lost all his warm season crops. All his tomatoes. All his flowers. All dead from the frost we had last week. Why him and not us? Is it our years of farming experience? Is it clean living? Is it purely happenstance? He farms on the same piece of land; there is no significant difference between where his crops are and where our crops are. The only difference that I can identify as a possible reason is our different mulching practices. He mulches with straw. This year, as I’ve mentioned a few times, we added plastic mulch to our farm practices. Could this be the difference? Let’s dive into the science to find out.
First lets look at straw mulch. Straw is the part of the grain plant that is not the grain part. It is mostly the stem part. It has a lot of volume relative to its mass. In fact, most of the volume and a little of the mass is just air. Air makes a good insulator, so it follows that straw would also be a good insulator, and it is! It is the goose down of mulches. Not only does it insulate the ground from the heat of the day/sun it also has a high albedo — the ability to reflect the sun’s radiation. In other words, it reflects a good amount of the sun’s energy that would typically be absorbed by the bare ground. Rich soil on the other hand has a low albedo so it absorbs more of the sun’s energy and heats up quickly due to its dark color. So by mulching with straw he reduced the ability of the sun to warm the soil below the straw and also reduced the ability of the soil to warm the air above it. Typically straw is a great mulch for reducing weed pressure, keeping the soil moist and reducing the soil temperature. Unfortunately in this one situation it appears to not be the best solution.
Now lets look at plastic mulch. Black plastic has a low albedo. It absorbs much of the sun’s energy. Where does this energy go? It has to either be stored or released. Or maybe, just maybe stored for a while then released? Let’s investigate.
As we all learned in our Heat Transfer class there are three types of heat transfer: conduction, convection and radiation. Conduction is the transfer of heat by direct contact of two bodies. Heat moves from the warm body to the cold body at a rate dependent on the properties of the bodies. A good example of conduction is the handle on the cast iron pan. When it is not being heated it feels cold to the touch. If you continue to hold the handle it will slowly heat up to your skin temperature as heat travels from your hand into the pan. But if it is being heated on the stove when you grab the handle the hotter cast iron pan will conduct heat into your less hot hand and you’ll end up with a nasty burn. Metals tend to conduct well. Insulation, not so much. That’s why they make oven mitts out of an insulative material and not a conductive one.
Convection is the second type of heat transfer. Convection is the transfer of heat through the movement of a fluid, again from the warmer to the colder body. Convection ovens work by using a fan to move the air (a fluid in the scientific sense) within the oven. By moving hot air around the oven the heat of the oven can be transferred to the cookie dough more efficiently. Natural convection can occur when a hot fluid is below a colder fluid. The hot fluid will naturally rise and displace the cooler fluid above. Weather is a great example of this.
Finally there is radiation. Radiant heat transfer is the heat you feel emitting from a warm body, like an electric stove or a fire or the sun. Radiant heat transfer does not require a substance to transfer the heat like the other two transfer types. It accomplishes it through the emission of electromagnetic waves. This is how the sun warms the earth with a virtual void between the two bodies. Without radiant heat transfer we would be colder than the planet Hoth!
So what does that have to do with black plastic? Well, black has a low albedo — the ability to reflect solar radiation, or shortwave radiation. So instead of reflecting the radiation from the sun back into the air like straw does, the black plastic absorbs the shortwave radiation. But what does it do with this shortwave radiation? Can it store it? Well, that’s where you have to reflect back to your thermal dynamics class. Black plastic has a low mass and therefore a low thermal mass. It really can’t store much thermal energy. But, there is something close by that has a high thermal mass — the earth! When we lay the plastic we try to get it tight against the earth and not flopping around all over the place. By putting these two bodies in contact we allow the energy absorbed by the plastic to transfer to the ground below the plastic using — back the heat transfer — conduction! If there is an air gap between the plastic and the earth the air can act as an insulator and reduce the ability of the plastic to conduct the absorbed energy into the ground.
So let’s walk through a day in the life of black plastic mulch. Let’s say it’s a typical warm sunny day with a cool frosty night. During the day the plastic absorbs the sun’s energy and transfers it to the ground below. It does this non-stop, doesn’t take a break, just keeps on absorbing and transferring until the sun goes down. Then at night when there is no more sun the air starts to cool. When the temperature of the ground is warmer than the air above energy flows from the ground to the air — again from hot to cold. It does this with some convection but mostly radiation. The plastic radiates heat back into the near ground air and short plants keeping both relatively warmer than the air above the straw mulch. So when we had the frost last week the plants planted on the black plastic were “protected” from the cold by a layer of air heated by the plastic’s radiation. Of course air moves and warm air rises so the energy stored during the days will slowly dissipate and at very low temperatures there is not enough energy to overcome the virtually unlimited cold air, at which point the plant may suffer frost damage. Who would have thought black plastic could be so useful?
In general we are big fans of straw mulch. It has a lot of very beneficial properties. It does the job of mulching very effectively and slowly adds organic material to the soil. It keeps the soil cool the way many roots and microbials like it. But it is risky to use on frost susceptible crops when there is a possibility of cold temperatures (the garlic in the picture above is not frost sensitive so the straw mulch was perfectly acceptable). As we’ve learned today, some of its best features can be used against you if you mulch too soon. Next week maybe we’ll take a look at which mulch is better when the temperature is in the 90s and there isn’t a cloud in the sky!
As always, feel free to send in questions, comments, suggestions, ideas, jokes or other flotsam or jetsam. And finally, the test: What are the three types of heat transfer? Wrong. There is also phase change heat transfer. Maybe we’ll cover that in a future discussion on our walk-in cooler.