Surfing the “green wave”

Is it spring yet where you are? How can you tell? Here in the Southeast, we are well along the path to spring, even though the calendar says we are still in winter. I can tell by the daffodils, spring peepers, and migrating birds I see overhead. I know those of you farther north may not be seeing any signs of spring yet, with winter storms still moving through your states and lots of snow on the ground as well as frigid temperatures, but trust me, it is coming!

© Jorge Royan / / CC BY-SA 3.0

What is phenology?

I first heard the description of the onset of spring as the “green wave” in “The American Seasons”, a book by naturalist Edwin Way Teale. It refers to the northward movement of the appearance of the first green leaf on bushes and trees as warmer temperatures move north and the days get longer. I find it to be a very imaginative and effective way of visualizing how spring moves from south to north (in the Northern Hemisphere) over the course of the season. Phenology is the study of when specific biological and natural events occur, such as seeing the first green leaf of the year, watching your forsythia bloom, seeing your local lake freeze over, seeing sandhill cranes fly north on their annual migration, or watching your favorite tree reach peak color in fall. Many of you probably keep track of these occurrences in your own gardens and use them to compare the climate from one year to the next. But did you know that there is a whole group of dedicated observers who have done this over long time periods and recorded their data for others to see and use?

Dietmar Rabich / Wikimedia Commons / “Dülmen, Vorpark, Narzissen — 2021 — 6901” / CC BY-SA 4.0

The National Phenological Network (NPN) is a group of dedicated citizen scientists and others who keep track of the yearly occurrence of when different indicators occur and report them to the NPN. Maybe some of you are part of this network!  They have an excellent database on their website with information for many different species of plants and birds as well as other interesting phenomena. You can explore it in a number of different ways, including through time series and maps. It helps to know the Latin names for the species you are interested in because different species respond differently to the weather! I even used it a couple of weeks ago to help a film director determine how long he had to shoot a Christmas movie before the trees leafed out in Georgia (response: do it soon!).

Where is the green wave now?

One section of the NPN site shows the 2022 movement of the green wave north with time and how it compares to the long-term average conditions. This week’s map is shown below, with areas later than average highlighted in blue and areas that are earlier than average in red. You can see that while southern Florida was ahead of normal, the green wave slowed up quite a bit later in January and early February as colder temperatures covered a lot of the region. That has switched more recently, with warm temperatures across the southern Plains showing the green wave reaching there about four days earlier than usual. Spring is also early coming to large parts of the West Coast, which is currently experiencing much warmer than normal conditions in most areas. If your area is not colored yet, you are still in the depths of winter, but keep watching and spring will (I hope!) be coming soon. I don’t know of a similar product in other parts of the world, but if any of you know, please share the information in the comments.

What do phenological records tell us about climate change?

While our local records in the United States are only a couple of hundred years old at most, other parts of the world have much longer records. Last year, Japanese scientists released a graph showing the change in the peak bloom date of cherry trees in Kyoto, Japan, for the year 800 to the present. While there are a lot of ups and downs over time, the trend towards an earlier peak bloom in more recent years is unmistakable. Since 1912, the average peak bloom date for the cherry trees in Washington, DC, has also shifted forward from April 5 to March 31. Other records showing the warming of the world include migration patterns of birds, pollen counts from trees, and ice-off dates on lakes in colder areas. Glacial ice and sediment cores from lakes and the ocean can provide timelines of how local biological systems have changed over time periods going back thousands of years. Many scientists are worried about the long-term consequences of these changes since not all species are migrating at the same rate and so some animals, birds, and insects may outrun their main sources of food if they move north faster than the plants that feed them.

Phenological records are important for monitoring long-term climate change because the records go back in time much farther than instrumental weather records do. Even though blooms and leaves on plants respond to temperature and sunlight in a non-linear way because they integrate all of the influences into one observed piece of data, they can still provide very useful information about how the environment is changing over time. A really interesting related use of this information was described recently in a story showing that the meteor that ended the Cretaceous period 66 million years ago probably occurred in spring due to the remains of fish that died in the devastating massive waves in the Gulf of Mexico that occurred after the meteor hit. Scientists assumed that the fish died immediately following the impact, and used their bones to determine that the fish were early in their annual growing cycle. Similar work has used buried vegetation to trace past tsunamis in coastal areas that may have been linked to other asteroid impacts or earthquakes that occurred before history was written down.

Wherever you are, I hope you enjoy watching the change in the seasons and in the world around you as much as I do. In spring, every day is a new adventure in seeing what is changing and hoping for the summer to come. I encourage you to keep a diary or other record of what changes are occurring in your garden so that you can see for yourself how the climate is changing from year to year.

Mt Hiei in Spring from Umahashi over Takano River. Source: Hahifuheho, Commons Wikimedia

The Gardens of Chernobyl 30 years after the disaster

Ukraine is all in the news these days as Russian troops are amassed along its borders in Belarus and neighboring Russia. I have some knowledge of Ukraine having visited the Chernobyl nuclear exclusion zone (the “Zone”) four times in 2012, -15, -16 and 2018. I had planned more visits but the global COVID-19 pandemic prevented my return to Ukraine and the Zone. The accident at Chernobyl was the worst nuclear accident in the history of mankind releasing more radio isotopes than the event at Fukashima and had long ranging impact on Ukraine and the then Soviet Union. Some say that the event precipitated the down fall of the former Soviet empire.

An image from the 2012 visit of the sarcophagus surrounding reactor IV of the Chernobyl nuclear powerplant.

Today the Chernobyl Nuclear Power Plant (CNPP) remains one of the most radioactive places that you can safely visit in the world. It was also the source of most of the world’s background radioactivity. When the disaster occurred in 1986, it temporarily raised the background gamma radiation of the entire planet by two percent. This rapidly declined as the half life of the released gases is very short and their radioactivity went away a few days and months later. Some of the elements, like radio Cesium 137 and Strontium 90, have longer half lives (around 30 years) and there was enough of them released to maintain high levels or gamma radiation where the fallout was most concentrated around the power plant. Elements such as plutonium remain radioactive for thousands of years but the amount of plutonium released was much less than that of strontium or cesium. Today the background gamma radiation near the CNPP remains up to ten times greater than the normal background levels found in the Ukraine capitol of Kiev. So how has this affected the gardens of Pripyat, the workers town not less than a km from the CNPP? To get to that let’s first talk more about the worker’s town and the disaster and then move on to what happened to the horticulture years later.

When the reactor exploded in 1986 (and yes it actually exploded) the area was surrounded with an exclusion zone complete with military checkpoints at 10 and 30 km circles away from the CNPP. This was an effort to keep people away from the radiation. The town that held the CNPP workers is Pripyat. It was a modern city of about 50,000 residents. It was considered the model city of its day as Chernobyl was considered the pinnacle of energy producing technology. Pripyat had a performing arts center, sports stadium, nightclubs, libraries several schools, public pool, and an amusement park. The entire population of the town was evacuated by bus in April 1986 in a few days. Although they were told they would return, most never did. Prypiat fell into ghost town status and remains that way to the present day, and like most ghost towns it was heavily looted. Some recent reports suggest that the Ukraine military has been knocking down buildings recently. Like any well planned city, Prypiat had an urban forest plan, street trees on every avenue and boulevard and gardens with ornamental plants. All were abandoned in 1986 and left to rainfall, radiation and the animals that remain there today.

A view toward the CNPP (on the horizon) from the roof of a 16 floor apartment building in Prypiat, Ukraine, note the verdant encroached forest.

The forest returned vigorously to Pripyat and animals roam the streets. The ecosystem recovery in the Zone has been dramatic over thirty years. Remnant street tree populations remain along the avenues but many more wild and non native exotics have invaded the spaces. The once athletic stadium playing field is now a small forest.

A forest grows on the end zone of the Pirpyat Stadium.

The forest encroachment has had a devastating impact on the architecture of the remnant buildings. Trees grow everywhere and when they attack the buildings they are able to collapse the floors and walls effectively demolishing the structure.

Trees began the demolition of this structure in 2016, a school building I had walked through in 2015.

People have great impacts on the health and structure of trees. When left alone they develop their own natural structure according to their genetic code. Over several visits we measured growth of trees in the Zone and took pictures to analyze their structure. We found that trees of similar age growing in Pripyat were smaller in size but had better branch structure due to LACK of pruning for 30 years.

Horse chestnuts in Kiev, Ukraine have been crown raised, have large pruning wounds, decay, and branch faults such as too many branches from one point and co-dominant leaders.
Horse chestnut trees along Lenin Boulevard in Pripyat have fewer branch faults and literally no pruning wounds after thirty years on their own.

It is hard to imagine what the gardens of Pripyat looked like at the time they were in cultivation because we have so few records of the city to review. There are the remnant street trees which my friend Igor Lacan studied extensively. Garden plant remnants are mostly gone except for extant rose bushes which can still be found around the city.

An extant rose plant in Pripyat, Ukraine.

It is hard to know what the gardens could have become before the forest invaded the city. We can look at landscapes in Kiev that exist today and see the overall gestalt of Ukraine gardens. They are kind of wild not meticulously maintained in public spaces but they also have charm, character and beauty.

A public park in the capitol city of Ukraine in Spring of 2018. Not a lot of maintenance but when in bloom full of beauty and impact.
Some color swirling through mowed weeds or “turfgrass” in a Kiev park.

As Ukraine is on the brink of uncertainty there are a few things that are certain, the radiation in the Chernobyl nuclear exclusion zone will continue, people will likely not be allowed to travel freely there, and the trees will continue to grow. The fate of gardens and trees in the capitol city of Kiev is less certain.


Lacan, I., J.R. McBride, and D. De Witt. 2015. Urban forest condition and succession in the abandoned city of Pripyat, near Chernobyl, Ukraine. Urban Forestry & Urban Greening 14.4:1068-1078.

Burlakova, E.B. and V.I. Naydich (eds). 2012. The Lessons of Chernobyl: 25 Years Later. Nova Science Publishers, N.Y.

Downer, A.J. and J.F. Karlik. 2019. “A Comparison of Two Horsechestnut Street Tree Plantings in Kiev and Pripyat, Ukraine.” Open J. Forestry 9: 255-263.

Karlik, J.F. and A.J. Downer. 2019. Comparison of Gamma Ray Dosimeters in a Field Study in the Chernobyl Exclusion Zone. J. of Air and Waste Man. 11:1361-1367

February is…

…National Pesticide Safety Month. Let’s review some key points of safe pesticide use. 

Socrates said, “ The beginning of wisdom is the definition of terms”

So let’s define a pesticide.
A simple definition is any substance used to control, deter, incapacitate, kill, or otherwise discourage organisms harmful to plants, animals or humans can be classified as a pesticide. A fuller definition can be found here. Germane to our discussion, herbicides make up 80% of all pesticide use. As gardeners we should know how to properly handle any chemicals we choose to use.

Anytime you use a pesticide, be sure to read and follow label instructions. The label will include important information for protecting yourself and it will tell you how to apply the product in the way that it will work best. Be certain the pesticide you’re using is correct for the job.

All pesticides carry labels which provide varying levels of information including the signal words “Danger”, “Warning’ or “Caution”. These signal words have specific meanings in relation to the pesticide. Products labeled “Caution” are the least toxic, “Danger” are the most. More information on signal words can be found here.

Correct and controlled application is responsible pesticide use. While some pesticides can be broadcast, e.g., pre-emergents and some lawn grub control products, most of them need to be precisely applied. Use correctly calibrated equipment recommended by the label directions and apply precisely. Avoid overspraying and watch out for drift.

And finally, wear protective clothing and use the correct application method and equipment as stated on the label. Always keep children and pets away while you’re applying any product. Observe wait times before allowing people or pets back into or onto treated areas. When you’ve finished application wash your hands, face and any skin that’s been exposed to the product. If needed, launder protective clothing separately from other clothing. 

For more information: