This morning I looked out and leaves on some of the trees are turning yellow. That hasn’t been the only signs that fall is coming. The geese are flocking up and flying by with their usual ‘good-bye’ honks.
The snakes are once again beginning to warm themselves in the midday sun because the evenings have been cool. The late season apples are beginning to ripen and I’ve put the blanket back on the bed.
If you believe the weatherman, some areas of our state will see frost next week. Many farmers and producers either dread the first frost or are doing an “I can’t wait until this is over because I’m tired” happy dance when it arrives. Most however just sit and wait and worry. There are things you can do to mitigate and predict frost in our area and it’s not too late to take action.
Above: A heavy frost formed on the stems of basil plants left in the field.
First, a few tips for those with high tunnels. Remember, your high tunnel will only give you about eight degrees of protection at best and at least maybe four to none – so don’t count on it to just do the job for you. How much higher temperature you have in the high tunnel will depend upon the height and size of your tunnel, its location and covering type, how well it is sealed up from drafts and wind, how cold it gets outside and for how long, and some inside environmental factors.
Above: The higher your high tunnel is, the more heat it will accumulate during the day and hold over night.
Moist soil holds more heat than dry soil. While I do not usually recommend wasting water on anything that doesn’t put a dollar back into my pocket, in this case it does. Water down all soil in the high tunnel well before the frost. A good morning soak on a sunny day before the frost is best. This allows the heat to build in the soil and act as a heat bank for that cold night. For food safety reasons we do not advocate for overhead watering of produce but I sometimes use a very small sprinkler and water just the walkways and end runs of rows and then use the soaker hoses for in the rows to ensure each area within the tunnel is moist. This goes for the field crops as well – doesn’t matter if it is inside the high tunnel or outside in the field – moist soil holds more heat. If conditions are right for frost (usually a clear sky) that is a good indicator that the day before has been a sunny one – get out there and use the heat to your advantage and water! In a study performed years ago, the air temperature above a wet soil was 5 degrees F higher than that above a dry soil and the difference was maintained until 6 a.m. the next morning.
Although the high tunnel provides protection, just like with your outside crops, tender vegetables in the structure might require an extra ‘blanket’ to keep them from frost damage. Although it’s much easier in the high tunnel in North Dakota because you don’t have to worry about the wind taking away your covering, there are still a few considerations. Be sure to use something that is not so heavy as to crush the plants. Even a light layer of Remay™ or other row covering can be just enough to hold a few degrees of heat.
It might be a pain in the behind to keep putting the wiggle wire or Z-strip on and off the high tunnel at this time of year, but if that is what you need to do to seal it up tight and keep out any drafts – then do it. Having a high tunnel is much like having a dairy herd – it is 24/7 while it is in operation and it requires constant monitoring. BE sure to roll down the sides of the high tunnel early each day – before sundown. You want a little sun or heat left in the atmosphere to build up in there to assist with overnight temperatures. Where I am at that is about 6 PM right now and getting earlier each day.
Speaking of monitoring – get yourself a couple of thermometers that track high and low temperatures. Farmtek carries some pretty simple to use ones http://www.farmtek.com/farm/supplies/ProductDisplay?catalogId=15052&storeId=10001&langId=-1&division=FarmTek&productId=22407 that we have used at the ECH and I have used in my own high tunnel. Having a thermometer so you know exactly how cold it got outside and inside the tunnel is very important. Keeping track of these numbers is just about as important. Make yourself a table on the computer or on a piece of paper and track the temperatures. In future years you will be much more versed in ‘prediction’ with some back up data. You will be more able to accurately predict what an outside temperature of 32 will do to the inside temperature in your high tunnel. You also want to watch your outside low temperatures because do you really want to rely on the news and weather report from 20 miles away to determine how cold it got? Temperatures and weather can vary greatly within even a mile of area – having your own thermometer that tracks highs and lows gives you accurate information and keeping those records from year to year assists you with choosing varieties, planting schedules and field growing locations.
Out in the field is also a place for you to do some predicting of your own. We all know that frost forms or collects ‘in the low areas’ but what does that mean? How low? To the naked eye, your field might look pretty level, but in all reality there are slight contours that air currents follow. One way to determine possible areas for frost in your fields is to use those leftover smoke bombs from the Fourth of July. On a still evening, light a few on one perimeter of your field. Watch where the smokes gravitates to. If it is a calm night and the wind or breeze is not blowing, the smoke should naturally travel along the ground and find the low areas. These are the areas that will frost first. Avoid planting sensitive crops in those areas – stick more to vegetables or crops that can take a little frost and be fine like carrots or kale or beets. Save the more sensitive items for the high ground.
What if you didn’t know this? What if you already have sensitive plants in the low spots? Even if you don’t – if you have a way to do it, moving air, such as with a fan, is a good way to provide just enough mixing of the air to keep the cold air from sinking and forming frost. I understand, this is NOT something you’re going to do with an acre of field but in certain smaller locations or for certain particular crops that you just want to get through one more night or one more week – using a fan may be enough to get you there.
To understand how this works we first need to look at some of the weather conditions that contribute to frost formation. First, a clear sky. A cloudy sky can act as a blanket and keep warmer air near the surface, helping with temperature.
Above: Clouds can hold in the heat from the day and help to prevent a frost.
Cool, clear nights with low humidity, often following a cold front, are signs of an impending frost. Next, a still night. During the day, Earth absorbs heat from the sun. At night, it radiates that heat back into the atmosphere, and cold air gradually settles around your plants. But a slight breeze mixes the somewhat warmer air from above with the cold air near the soil. So frost is less likely on a night with a gentle breeze (but not strong wind) blowing. Next, the dew point. As the evening temperature falls, the air holds less and less moisture, until it condenses and dew forms. The temperature at which this happens is called the dew point. When dew forms, heat is released. That heat helps to keep the air temperature at or slightly below the dew point. So, the more moisture in the air at sunset, the less the likelihood that frost will occur during the night. (This is why commercial growers turn on sprinklers when frost is predicted. The added moisture in the air raises the dew point.) The dew point is also a fair indication of how cold it will get that night.
Some forecasters give the dew point in their evening reports. Be sure to check the dew point at or after 6 PM. Dew points change throughout the day and to get the best estimate on what the dew point for that evening will be check after 6. You can also check online at our National Weather Service site or Facebook page or at the North Dakota Agriculture Weather Network site. All three links have been posted here.
North Dakota Agricultural Network: http://ndawn.ndsu.nodak.edu/
National Oceanic and Atmospheric Administration – use this link to find the location nearest to you for information: http://www.legislative.noaa.gov/NIYS/
So let’s do what we can to make this season last as long as possible before we’re dealing with what eventually will come…