January 2023 – The Garden Professors™

Chill out! Winter cold and chill hours for fruit

If you have fruit trees in your garden, then you may already be aware of the importance of winter cold for the development of blooms and fruit. Some fruit varieties like apples, cherries, peaches, and blueberries all need a certain number of hours of temperatures below 45 F to prepare the trees to bloom in the spring. Surprisingly, other trees you might not think of as fruit trees also need a period of cold conditions to produce a good crop, including pecans and olives. Many other garden plants also require cold periods to provide the best blooms, such as lilac and hydrangea. I miss seeing lilacs here in the Southeast because we just don’t get cold enough to meet their chill requirements, although new varieties that can flourish in warmer conditions are being developed.

Closeup on small pink flowers of a Syringa pubescens subsp. microphylla ‘Superba’ at Hulda Klager Lilac Gardens in Woodland, Washington. Dcoetzee, Commons Wikimedia.

Why do some plants need winter cold?

The cold conditions over winter cause the plants to go into dormancy. That helps protect the plants from harsh conditions over the winter. Once the plants experience the number of chill hours required for that plant, they are ready to begin the blooming process once the temperature warms up. If the fruit trees do not get enough hours of cold over the winter, they do not bloom well in the spring and blossoms may be delayed or bloom at irregular times or not bloom at all. Leaf emergence may also be affected. The potential yield of fruit is reduced due to the lack of a strong uniform bloom.

Scientists count the number of hours a tree or orchard is below 45 F and measure the accumulation of those hours as “chill hours.” Some publications call them “chilling hours” instead. They are usually accumulated starting around October 1 and go through the winter into the spring bloom period when bud break occurs. An alternate method for calculation only counts the hours between 45 and 32 F. Chill hours are different from cold hardiness, which is the lowest temperature that a tree or plant can tolerate without dying. Recent research has allowed scientists to develop more sophisticated methods that dynamically calculate “chill portions”—these calculators take into account variations in temperature over time and can reset the calculation if cold conditions return after a warm spell.

Peach orchard in Virginia, United States. Karen Blaha, Commons Wikimedia.

Picking the right fruit variety for your garden

Most types of fruit other than citrus and each variety has a preferred minimum number of chill hours for that variety to set a good fruit crop. If you live in the Southeast, you may be able to plant varieties of blueberries that require only a few hundred chill hours, where in the north you may be able to plant a more cold-hardy variety that requires over 1,000 chill hours for the best bloom. Because of the cold outbreak we had this December, some of our blueberry varieties are already blooming this year in the warmer weather we have had in January.

If you are planning to plant a fruit tree in your garden, you will want to pick a variety that matches the expected number of chill hours for your area. If you plant a variety that expects a lot of chill hours but your location gets only half that amount, you will not get very good yields from your trees since the plants won’t break dormancy correctly. But if you pick a variety that requires a low number of chill hours for your area, then the tree will be ready to bloom as soon as the chill hour requirement is met, which could put them at risk for a frost if they bloom too soon. So picking the right variety for your area is crucial! Most nurseries can provide the recommended number of chill hours for the fruit variety you pick.

In the future, as temperatures get warmer under the influence of greenhouse warming, most areas will see a decrease in the average accumulation of chill hours over time. We are seeing this at my weather network stations in Georgia and in many other states as well. If you plant something that has a long lifetime, you may want to plant varieties that require fewer chill hours than your current climatological average to make sure they will thrive in a warmer climate in the future.

Where can you find information about chill hours for your location?

For average numbers of chill hours for the United States, check out the map below. Keep in mind that your local microclimate will affect the number of chill hours your garden will receive.

Here are some sources of current chill hours for this year:

The Georgia Weather Network has a tabulation of stations with current accumulations since October 1 at https://weather.uga.edu/aemn/cgi-bin/AEMN.pl?site=AAAA&report=ci. Many other state agricultural weather networks provide similar information. You can find a list of many of them listed under the Partners tab at the National Mesonet Program website.

The Midwestern Regional Climate Center has a current map for the contiguous U.S. similar to the at https://mrcc.purdue.edu/VIP/indexChillHours.html. An interactive version based on GIS maps is at https://mrcc.purdue.edu/gismaps/vipstndata.htm.

AgroClimate has a chill hour calculator for Florida and Georgia at https://agroclimate.org/tools/Chill-Hours-Calculator/ which allows you to choose the chill hour model and the time period of interest.

Cherry blossoms near Fukuoka Dam, Tsukubamirai, Ibaraki, Japan, March 2018. t.kunikuni, Commons Wikimedia.

Don’t forget to check out our archives!

If you are interested in this topic or have other questions about gardens and gardening, we encourage you to explore our archive of blog posts to see if you can find the answer to your questions. Here are a few of the previous stories about winter chill you might find interesting:

Chill hours and bud break in Christmas trees:

https://gardenprofessors.com/and-now-for-something-completely-different/

https://gardenprofessors.com/the-walking-dead-christmas-tree-edition/

Chill hours and lack of flowering in landscape plants like lilac:

https://gardenprofessors.com/why-doesnt-my-plant-flower-part-1/

Another “drainage solution” that makes problems worse

**Lack of surface drainage suggests problems below ground**

I received an email this week from an arborist colleague who had been sent an “engineering solution” which claims to help with rooting issues in clayey soils or areas where root area is reduced. There was a spiffy diagram accompanying this which I’ve reproduced below.

I could dissect this for you and point out all the problems right now, but instead I’d rather supply you with some factual information and let you apply it to this “engineering solution.”

Planting hole material that is not the same as the surrounding soil will have reduced water, air, and root movement due to the abrupt changes in texture. The hatched material in the pit appears to be different from the surrounding soil, leading to the assumption we’ve got modified backfill. Here’s a peer-reviewed journal article that discusses the fallacy of soil amendment.
“Augured sump drain/root channel bores” are simply modified French drains. French drains serve to move free water (i.e., water that is not in soil pores) somewhere. Where “somewhere” is in this case is unclear.
French drains and other drainage systems do NOT reduce the amount of water that soil holds. Field capacity is the term used to describe a saturated soil. A sandy soil has a low field capacity and drains quickly. The higher the clay content of a soil, the higher the field capacity and the slower the drainage.
“Drainage material” placed beneath the root zone will slow water movement and create a perched water table above the “drainage material.”
Drains as well as soils that are full of standing water have no oxygen. Roots will not grow where oxygen is unavailable.

I don’t think I need to belabor these points any further. The bottom line is that you are going to create textural discontinuity problems in the planting pit if you follow these guidelines.

Home Greenhouses Pt. 2: Regulations & Restrictions

Last month I dipped into some of the considerations of owning or building a home greenhouse, having just “inherited” a greenhouse with my new house. The questions and comments were numerous, so I decided to continue on discussing considerations for home greenhouse ownership and operation. There will likely be a few more articles down the line….so strap in.

This time around, I thought I’d talk about something that we hate to talk about, but is important to understand: rules and restrictions around what you can and can’t do, especially if you live in an urban area or municipality.

I know, I know. We all just wish we could do whatever we want, but when we live in close proximity to others there are usually some sort of rules we have to follow to keep the peace. I have a little better understanding of how it works, now that I helped write an urban ag ordinance with our city planning department (fingers crossed is passes city council next month!) that will “decriminalize” urban farming, including controlled environment ag structures like greenhouses. Of course, aside from what you can and can’t do there are tax implications that you should be aware of, no matter where you live (more on that in a bit).

Zoning, Ordinances, and Planning Commissions – oh my!

Most agricultural activity, which includes greenhouses, is likely regulated to some extent if you live within a municipality. Even if you don’t live in a town or city, there could be certain building codes or ordinances on county or state levels that you’ll want to check into.

You’ll want to look up your municipal code online, or call your municipal administration (like city hall, mayor’s office, planning office, etc.-whatever you have) to see if there are specific allowances or restriction for or against greenhouses. There are a few scenarios that could play out here, so you’ll want to be prepared on how to proceed. In general, you might find that greenhouses are:

Permitted, but only in certain zoning areas (more on that later)
Permitted, but requiring a permit of some sorts
Restricted altogether
Not mentioned at all

If you fall in that last category, then anything you do will exist in a gray area, where it isn’t strictly legal or illegal but the fate of your greenhouse (or any fines you might incur) would be up to interpretation by whoever is in charge of compliance (and what side of the bed they woke up on that day). And your greenhouse could be made legal or illegal at any time in the future if some sort of ordinance or decision is made. In the case of our new ordinance here in Omaha, greenhouses and structures like high tunnels will be allowed, but only in certain zoning areas. People who already have structures (and have for years) will now find that those structures are either allowed or banned (or will require a permit). Of course, enforcement of issues like this are often complaint-based, meaning that the municipality probably isn’t driving around looking for your greenhouse but if you and your neighbor aren’t on good terms they can call and turn you in.

As for zoning, there are several different categories and sub categories. The ones you’re most likely to encounter are residential, commercial, industrial, and agricultural (there are others as well). And then there are usually numeric (or named) sub-zones within those zones denoting the density or intensity of use on those zones.

For example R-1 (Residential 1), might be for a single-family dwelling whereas R-3 would be for a small apartment building. C-1 (Commercial-1) might be for a single-use building like a stand-alone fast-food restaurant and C-8 might be for a skyscraper office headquarters. Municipalities use these codes to denote what is and what is not allowed in certain places. For example, the code we worked on for Omaha allows certain types of farming on all residential zones with a conditional use permit and on commercial zones without a permit. (Home gardens and even community gardens are not considered farms and are allowed on all residential zones without a permit.) Many municipalities may also have agricultural zoning on properties around the periphery but still within the city/town limits. On lots like this, most agricultural practices are allowed.

Three things you can do with NYC's new zoning and land use map - Curbed NY — Your municipality may or may not have a map available showing lot zoning, like this small area of NYC.

Since a greenhouse is a structure, there may or may not be a requirement for a permit for the structure, even if it is allowed. This could be dependent on a number of factors, including whether it is considered a permanent or temporary structure, whether there are utilities going to it (which is usually the case for a greenhouse), or even how the structure is constructed. In some areas, it comes down to how pretty or ugly the city and your neighbors think the structure might be. In discussions for our ordinance, there was talk of not requiring a permit for a greenhouse but requiring permits for structures like high tunnels/hoophouses because there’s an (incorrect) assumption that greenhouses are prettier than high tunnels. But many people make greenhouses out of those structures (the greenhouse I inherited is just a high tunnel with a space heater). There was also an assumption that a greenhouse wouldn’t have loud, flapping plastic but a greenhouse would (I kid you not).

The tax man cometh….for your permanent agricultural structure.

One other thing you’ll want to consider about a greenhouse has to do with the tax bill. Since greenhouses are considered permanent structures, many places consider them a permanent improvement to your property and will add them to the tax bill like they might do an out building or garage. The municipality will definitely know about it if you have to apply for a permit or if you get a visit from the assessor. But this could also be dependent on how your structure is built. If you build a solid sided greenhouse (one of the nice looking ones) with a concrete slab floor, permanent utilities, etc then it will definitely considered permanent. But if your structure is more like a high tunnel/hoophouse like mine (a high tunnel in a trench coat pretending to be a greenhouse) then it could be considered a temporary structure and not taxable.

My high tunnel parading around like a greenhouse

High tunnels are considered temporary since you could really take them down at a moment’s notice (and some planning department officials think this means that you take them down and put them up each season, despite how much an extension educator tries to convince them otherwise….but I digress). So your mileage may vary depending on your local rules and how your structure is constructed.

So what home greenhouse topic should we cover next? I’m sure there’s plenty of questions out there – be sure to leave them in the comments. Also – would you like to have your greenhouse featured in a future article (this means I don’t have enough home greenhouse pictures)- feel free to send them to me at john dot porter at unl.edu.

Companion plants, they are not what you think!

Companion plants! Great, what a good idea. When you first hear the term and think about the concept it sounds great but there is a lot to not like about it. The term “companion plants” implies that these plants are partners and they “enjoy” each other’s company. The term is an anthropomorphism or overlaying of human qualities on non-human organisms. A more appropriate term may be plant associates or plant associations, a term taken from plant ecology, which has more basis for use.

Plants naturally grow together in groups which are called plant communities. These plants evolved under certain climate, soil, and environmental conditions that allow them to live together in the same place. Coastal sage scrub, oak woodland, and juniper pinyon woodland are some common plant communities where I reside in Ventura County. All of the plants growing in these communities receive winter rainfall and summer drought (Mediterranean climate) to which they are adapted to grow in. Plants growing here either resist drought through specific plant adaptations such as reflective leaf surfaces (white sage), abundant trichomes (sycamore), microphylly (buckwheat), succulent water storage (agaves, yucca and other lily family bulb forming plants), and C-4 metabolism (grasses). Some plants avoid drought by growing in the rainy season, setting seed and then remaining dormant during hot dry weather. Plants can grow in this climate because they have the adaptations to do so.

Plants compete for resources and while doing so may provide a place for other plants to grow. Trees have an advantage over grasses because they can grow above, catching the sun and shade the grasses out. But shade may also provide a place for shade adapted plants to grow. Plants surviving challenges in a specific environment may end up growing together. Woody plants also provide perching birds a place to defecate and spread seeds. This is why unexpected things may grow under other plants. Shade may even be necessary for development of proper form. We have noticed in studying western hackberry (Celtis reticulata) that the tree has no apical control and will not develop into a tree shape when grown in full sun. When grown in shade apical control is present and the plant grows a straight trunk. Birds commonly eat hackberry fruit and likely disseminate it under the canopies of other plants. I don’t think the hackberry minds growing as a blob but its “companion” plants cause it to change form due to changes in light intensity.

*Celtis reticulata* growing in full sun has no apical control and sprawls as a giant bush yet in nature when it germinates in shade, it develops a straight trunk with fewer branches.

Some plants live very closely with others. Mistletoe is a great example. Leafy mistletoe is a hemiparasite deriving its energy from sunlight of its own leaves but utilizing water and photosynthate from its host. Similarly there are free living plants such as Indian paint brush (a member of the Orobanchaceae) that are also hemiparasitic using their roots to extract benefit from neighboring plants. Holoparasites are true parasites deriving all their nourishment from their hosts, e.g., Dodder (Cuscuta spp.). Dwarf mistletoe is also holoparasitic as it largely lacks chlorophyll. These plants are always found on or near their hosts but it is hard to call them true “companions.” The plants clearly associate with each other and in some cases are detrimental as one of the plants stands to gain nothing from the interaction.

This Indian paintbrush is a hemiparasite. It can be free living or associate with other plants and use their photosynthate.

One popular example of “companion planting” is The Three Sisters (TTS) polyculture of corn, squash and beans. This agricultural system is said to be synergistic. Corn provides support for beans and shades the squash, and beans provide nitrogen fixed from the air for the other two members of the system. The system was “practiced” by indigenous Americans all across the continent. Soils, rainfall and climate are quite diverse across the United States, and I am sure that TTS agriculture had mixed success. It is an interesting thought that the human diet can be satisfied by these crops and likely the combination was more about ensuring sustained calories and nutrients for those who grew them. In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.

Mutualism is the concept that interactions between two organisms benefits both. There are many examples of plants that have a mutual relationship with insects, birds, fungi and bacteria. I found no examples of plants that have mutual relationships with other plants, e.g., “companion plants”, common to the scientific literature. I thought this was unusual so I called a friend who is a plant ecologist and asked her the question. At first she was enthusiastic and pointed to non-plant-plant relationships. As I redirected her to only plant-plant interactions we could not identify anything. My suspicion is I have missed something important or we will discover one day that there are plants evolved to help one another but for now, it evades me.

There is no doubt that one plant can help another but it’s incidental and not a sign of a mutual relationship. Most plants evolved to grow in communities because the growing conditions are suited to all. Knowledgeable gardeners and landscape architects will group plants that grow well together. This is only common sense.
Understanding how plants grow in nature informs gardeners about adaptations and this in turn elevates the practice of horticulture.

In this image agaves grow at the base of an Alligator juniper, very companionable. But is there a benefit for either plant?

Agaves like to grow next to rocks. My son’s theory is that both rocks and trees protect the agave from being eaten by javelina. Or perhaps there’s just more moisture under the stones?

References

Martinez, R.T. 2008. An evaluations of the productivity of the native American ‘Three Sisters’ agriculture system in northern Wisconsin. M.S. Thesis. University of Wisconsin-Stevens Point, College of Natural Resources.

Marsh, E. 2023. The Three Sisters of Indigenous American Agriculture. National Agricultural Library (USDA). https://www.nal.usda.gov/collections/stories/three-sisters