The Tucson-based National Phenology Network has gathered a lot of data about the lifecycle of plants and animals and created some powerful data-reporting tools to help farmers and scientists.

Now the organization would like citizen observers to help it provide meaningful information for your average backyard gardener.

The possibilities are intriguing, from determining when to plant seeds to anticipating when pests will appear.

By using these tools, “we can give folks advanced warning” for all sorts of gardening milestones, says Theresa Crimmins, the network’s assistant director.

You can tell when flowers will bloom, when leaves will start to grow on a seasonally dormant plant and when fruit is ready to harvest.

What the group’s tools and data do is provide predictions based on a combination of current temperature and historical weather information.


It starts with accumulated growing degree days, also known as GDDs, degree-days or heat units.

A heat unit comes from a mathematical formula using daily high and low temperatures. It gives a value to weather in a specific place.

The heat unit takes into account the unique conditions of a place in a specific time period. The information shows the effects of cold and warm weather systems that cause differences compared to average temperatures.

Let’s say that for Tucson, Day 1 had five heat units. Day 2, a warmer day, had seven heat units. The accumulated heat units for those two days are 12 (5+7). Day 3, a cooler day, had one heat unit for an accumulated GDD of 13.

Cold days will have few if any heat units, while warm days will have high numbers of heat units.

Each plant and animal hits lifecycle milestones when it reaches specific heat units.

For instance, New Mexico-type chiles will reach peak bloom at 2,000 heat units and fruits will ripen for harvest at 3,200 heat units.

That’s according to a July 2013 “Heat Units” report by Paul W. Brown, an extension specialist with the University of Arizona’s College of Agriculture and Life Sciences.

If you were growing Hatch chiles, you would know that you can expect that the most flowers will appear when heat units reach 2,000 in Tucson. A table can tell you what date that will happen.

Depending on weather conditions, that date could be different from the 30-year average. Unusually cool weather could result in taking longer to reach 2,000 GDDs. Unusually warm weather means 2,000 GDDs could come sooner than expected.

The National Phenology Network, as well as the University of Arizona, have tables of heat unit data updated daily.

Calculating heat units and applying it to the calendar is complex, says Peter Warren, an urban horticulture agent with the Pima County Cooperative Extension and Arizona Daily Star columnist. Backyard gardeners use planting guides instead.

“Plant guides...are often based on grower experience rather than GDDs,” Warren says in an email. “The risk of using this type of static that the plant guides aren’t accurate every year due to fluctuations in the weather.”


The phenology group uses colorful maps to show these fluctuations, which can be dramatic.

By examining the GDD maps on its website, Crimmins was able to determine that the accumulated heat units for March 1, 2016 in Tucson was the same as the 30-year average of heat units for mid-March.

“Conditions had been warmer than average between Jan. 1 and March 1,” she says. “It seems that folks could have planted about two weeks earlier this year than in an average year.”

All of this may sound pretty wonky, but Warren says it’s helpful information for your own garden.

Take pest control.

“Since we know the rate of insect growth is temperature dependent, tracking degree-days can help us predict the window of potential insect activity,” he says.

Knowing when a bug will start to do damage on plants will help you know when you need to act.


Crimmins and Warren agree that there aren’t enough data for home gardeners, particularly those in Southern Arizona, to take advantage of this more precise way to garden.

Backyard pests are not well researched, says Warren, who has an interest in insects. But if gardeners watch what happens in their gardens, they can figure this out.

“The calculation of degree-days along with careful observation of insect activity can be important information to record as part of an integrated pest management plan,” he says.

“Information gathered can be used in future years to better predict upcoming pest problems and to be better prepared to manage them.”

Phenology is all about observing plant and animal activity to track lifecycle changes over time.

Some home growers do this by filling out gardening journals or making notes on seed packets about how a plant is progressing through the growing season.

A similar recording can be made for the National Phenology Network using its online Nature’s Notebook.

You can’t include your own activities such as when you planted or watered, but you can see when the milestones occur and how that changes over time.

The notebook application turns your entries into a timeline that shows when on the calendar your plants are hitting milestones.

At the same time, the information you put into your notebook is added to the organization’s database that would be available to the public.

Once data on common landscape and crop plants in Tucson are added, they can become a useful resource, says Crimmins.

She sees a time when a person can look up calendars that apply to one’s own neighborhood, then refer to the heat unit tables to figure out important milestone dates.

“There is a ton of potential,” she says.

Contact Tucson freelance writer Elena Acoba at