Spring Soil Temperatures

By John Wallace, SIFT Farm Manager

The biggest question I ask each year is, how early is too early to begin direct seeding in the ground?. Inclement weather will persist through the beginning of June, making it difficult to keep air temperatures above freezing. However, the soil temperatures are only going to get warmer as we move through the end of April. On April 27, 2018, I took soil temperatures within the hoop houses and the outdoor plots. Having had only a handful of warm sunny days, the hoop houses are already doing their job of heating the soil within. By taking soil temperatures consistently in all the hoop houses, we can get a better idea of when it is safe to begin direct seeding.

Soil temperature of 62°F in hoop house 5 on April 27.
Soil temperature of 62°F in hoop
house 5 on April 27. Photo: NCAT
Soil temperature of 44°F in outdoor plots on April 27.
Soil temperature of 44°F in outdoor
plots on April 27. Photo: NCAT

Using a compost thermometer, I was able to read the soil temperatures more than a foot below the surface. The outdoor plots were the coolest, as expected, measuring around 45°F. The raised beds were slightly warmer at 49°F. The smaller hoop houses, which have proven to be less effective than hoop house 5, were measured around 52°F to 56°F. One reason for the wider range is that the soil near the perimeter was much cooler than the soil in the middle. We have noticed that hoop houses with a length:width ratio of 2:1 hold a more consistent temperature. Hoop house 5 is much larger, and the length:width ratio is closer to 2:1. In addition, it has a double polyethylene film that heats and insulates much better than the others. The soil temperature inside hoop house 5 ranged from 59°F to 62°F, topping the list as the warmest soil. The day these temperatures were taken, it was sunny and 72°F, yet the closed-up hoop houses produced inside air temperatures that were 10°F to 16°F warmer.

The soil temperature is a direct indicator of how long it will take for the seeds to germinate. If the seeds do not emerge quickly enough, they can end up rotting in the soil. Seeds from spinach, lettuce, and turnip will germinate rather quickly even when the soil as cool as 50°F. Cucumber, watermelon, beans, and peppers would not likely germinate at a temperature that low. There are also seeds that can germinate in soil temperatures down to 32 degrees; however, the amount of time it would take to germinate would be measured in months rather than days. Examples of seeds that can manage those temperatures are onions, spinach, and parsnip. We have tried different methods of overwintering and early spring seeding just after the ground defrosts, with mixed results. Nonetheless, a good amount did germinate despite the harsh conditions. For a great resource, visit this Oregon State Extension webpage to view a table on days to appearance of seedlings at various soil temperatures.

Air temperature reading of 98°F in hoop house 5.
Air temperature reading of 98°F
in hoop house 5. Photo: NCAT
Air temperature reading of 82°F in hoop house 3.
Air temperature reading of 82°F
in hoop house 3. Photo: NCAT

Based on our crop rotation, hoop house 5 is planned to grow beans, squash, and cucumbers. These are all warm-season crops that will not grow well in cooler soil temperatures. Around 60°F is as low as we’d want to go for beans. They would ideally prefer soil at 75°F. Our plan is to go ahead and begin preparing the beds for the beans, knowing that we are nearing the ideal temperature for direct seeding. To speed up that process, we will then set up a caterpillar tunnel within the hoop house to double up on the greenhouse effect. This will hopefully give us a head start on the beans and guarantee strong yields before fall frosts return. Previously, the beans were grown in one of the hoop houses that does not guarantee as much protection as hoop house 5. Last year, we did not seed the beans until mid-June when we were past the potential for possible frosts. We got two weeks of harvestable beans before they were frozen solid by 20°F weather at the beginning of September. Knowing that this year the beans will be equipped with our best defenses, we aim to have the beans seeded nearly a month earlier than was possible last year.

Posted on: May 7th, 2018