Models for




By NICK ANDREWS | Oregon State University Extension

LEN COOP | Oregon State University Dept. Horticulture and Integrated Plant Protection Center

STEVE ELLIOT | Western SARE and Western Integrated Pest Management Center

HEIDI NOORDIJK | Oregon State University Extension

HEATHER STOVEN| Oregon State University Extension


Pest managers are familiar with the concept of using degree days to predict

pest outbreaks. Insects, plant diseases, and other ectothermic organisms like

crops and weeds, develop in relation to the temperature around them, and

degree-days are a way to measure accumulated temperature over time. Degree-days

are usually a more accurate way of predicting their development than calendar time


The team at Oregon State University (OSU) is adapting a degree-day modeling

system built for pest management into a tool for vegetable growers to better plan

their planting and harvesting dates and prevent weed seed rain. It’s called Croptime

and growers throughout the United States can use it at

Croptime is a degree-day-based tool that overcomes a common shortcoming of

seed catalogs, which give expected maturity dates in calendar days or other rough

estimates of time-to-maturity. Calendar days are pretty inaccurate, and growers

recognize that. Days to maturity depend on the time of year and location the crop

is grown in. For example, various catalogs report that ‘Arcadia’ broccoli matures in

65 to 90 days. That range is consistent with the team’s findings in the Willamette

Valley in western Oregon, depending on when and where it is planted. Degree-day

models can help you predict where you fall in that range, usually with an accuracy

of a week or less.

Degree-days are more accurate than calendar days, and are especially useful when

crops are planted early or late, or when the weather is unusual.

Croptime was built on the backbone of the agricultural and pest

management degree-day modeling site that is linked to thousands of individual

weather stations throughout the U.S. If you use the degree-day models in areas

with a different climate than the Willamette Valley, remember that the models

were developed in the Willamette Valley, Oregon, and use some caution. Degreedays

are usually more accurate than time, but other factors like day-length, light

intensity, water availability and pest pressure can also influence time to maturity.

For example, the Willamette Valley doesn’t get many days with high temperatures

above 90°F. So the team can’t identify maximum daily temperatures for crops like

tomatoes and peppers that could be important in hotter climates.

The site currently has about two dozen vegetable models, including four varieties of

broccoli, seven cucumber varieties, four sweet pepper, four tomatoes and four sweet

corn varieties. It also has models that predict viable seed set for redroot pigweed,

lambsquarter and hairy nightshade.

Continued on Page 26

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