Weather monitoring is important for understanding crop conditions, determining the timing of pesticide application, and deciding whether previous pesticide applications were effective. However, it is important to understand how weather conditions should be monitored so the information is accurate and consistent from one event to the next and one location to the next. The two most important weather variables to monitor are rainfall and air temperature.
Wind speed is also important for understanding pesticide drift, but is not the focus of this article since it is best collected at the time and place the pesticide is applied using a hand-held device.
Most growers monitor for rainfall because it is important for understanding the need for irrigation and determining if previous or current pesticide applications were or will be effective. However, not many people know how to accurately collect rainfall measurements. A rain gauge is all that is necessary to collect rainfall data, but the size and placement of the rain gauge is important for determining its accuracy.
Official rain gauges typically have a diameter of 20 cm (8 in.), which gives a much more accurate rainfall total than a rain gauge that has a 2.5 cm (1 in.) diameter (Figure 1). This is because a small rain gauge can be more influenced by one or two large rain drops that happen to fall into the hole.
Figure 1. The rain gauge on the top is automated and is much more accurate due to its much larger opening. The backyard rain gauge on the bottom is a little less accurate due to its smaller diameter, but would be accurate enough if it wasn’t placed as close to trees in the area.
The main issue with collecting rainfall measurements is splashing from nearby obstacles. Raindrops splash considerably off flat surfaces. This includes the ground, standing water, a railing, leaves and branches of trees, and shade structures (Figure 2). Since rain gauges are long cylinders, raindrops are not likely to splash water out of the gauge. Any splashing of water into the gauge will always skew the measurement to be too high.
Rain gauges are best placed over mowed grass to reduce splashing and the rim of the gauge should be at least 60 cm (2 ft.) off the ground. When it is windy, obstacles can also block some of the rain, so it is recommended the gauge be placed away from obstacles by twice their height (including shade structures). Avoid placing them in a windy location, which can also skew the measurements.
If you prefer to place it near the field to get the most accurate information for your crop, place it at least 60 cm (2 ft) above the vegetation on the edge of the field if there are no trees in the area, or extend a pole above the shade to mount the gauge and ensure the rim of the gauge is at least 60 cm (2 ft) above the shade cloth. This may be more difficult to empty easily. Rainfall measurements taken with the garden are useless, since the shade cloth results in highly variable distribution of water, as can be seen by the drip-line chlorosis that usually develops by mid-summer.
How Much Rainfall is Sufficient?
The most common question I get is how much rainfall is enough to wash an applied pesticide through the straw to the root zone. This is not easy to determine remotely and depends on straw thickness and how dry the straw was before application. There are two methods to determine this in the field:
- Check it by hand by removing the straw in a spot during or immediately after the rain and determining if the soil is saturated at ground or root level.
- Purchase a data logger with a soil moisture sensor or two attached. One can be placed under the straw at the ground level, and one about 15 cm (6”) down. Both should be at a 30 to 45o angle to allow any moisture to drain off the sensor after the rain is done. The loggers can be set up to be viewed remotely or downloaded rapidly onto a phone. When you see the moisture spike, then the irrigation or rainfall reached that level in the soil.
Air temperature is less variable by location than rainfall, but most growers have a thermometer to monitor temperatures. Temperatures can determine if a pesticide will burn the foliage, whether a garden needs extra air-flow or if a frost event is likely to occur. While most people have thermometers, very few have their thermometers located in a way that collects the temperatures accurately.
Official temperature readings and daily forecasts are based on air temperatures at around 1.5 m (5 ft.) off the ground over a mowed grass surface and away from any obstacles by at least twice their height. The thermometer is typically placed inside a wooden, louvered structure that is painted white called a Stevenson screen (Figure 3). This allows air to flow through but blocks the thermometer from the sun. Any sun that hits any type of thermometer will heat it up much higher than the actual air temperature. The white, wooden structure reflects most sunlight and does not allow heat from direct sunlight to be easily transferred into the structure.
Figure 3. This weather monitoring station contains a thermometer inside a smaller plastic version of a Stevenson screen on the left. Although it is not placed properly for an official temperature reading (i.e. mowed grass, away from trees), it is suitable for monitoring temperatures within the orchard.
Thermometers placed next to a north-facing window may be somewhat accurate on a breezy day but will be cooler than the actual temperature on a calm sunny day and warmer than the air temperature on calm, cold nights, since the heat radiating off the house will warm it up. If the sun hits the thermometer at any point, it will dramatically exaggerate the temperature by up to 10 or 15 degrees.
For ginseng, we only care about temperatures within the plant canopy, but those can be highly variable depending on where you are in the field. Temperatures will be much warmer in the interior of the garden and at the top of a hill within the garden. On a windy day, air temperatures outside and inside the garden will be closer than they will be on a calm day. Outside air temperatures are more consistent and give a better indication of field conditions than any one reading taken in the garden. Multiple temperatures taken at various points throughout the garden would be ideal, but most growers do not have that set up.
It has been shown that air temperatures under the shade on a relatively calm, sunny day can be up to 6oC higher than the ambient air temperature. Keep this figure in mind when deciding on when to apply a pest control product based on air temperatures outside of the garden.