February 2023 – The Garden Professors™

Are we in winter or spring? The answer is YES!

If you’ve been following the national weather this week, you might be wondering if the groundhog has developed a split personality this year. Is winter over or are we in for six more weeks of cold? While the eastern half of the United States is feeling the effects of record-setting high temperatures and one of the earliest springs on record, the western U. S. is observing cold and snowy conditions all the way down into Southern California. I’ve heard reports of snow reaching all the way down to Tucson in southern Arizona.

A bee on a yellow flower

Description automatically generated — Close-up of forsythia, Cambridge MA 2019, Burneraccount22. Source: Commons Wikimedia.

What is an atmospheric wave?

Some of you may be wondering how we can have some tremendous differences between the eastern and western U. S. at the same time. The reason we see such differences is linked to the wavy pattern of the upper atmosphere, which helps direct warm or cold air into different regions. Today I will discuss atmospheric waves and how they affect the surface weather.

Since air is a fluid, it should not be surprising that it has waves in it. In fact, there are many different types of waves that can occur in the atmosphere, ranging from small-scale pressure waves downwind of mountain ranges to the largest planetary-scale oscillations in pressure. Sometimes you can see some of those smaller-scale waves in clouds occurring above you.

Wave clouds in Algeria, 2006, Pir6mon. Source: Commons Wikimedia.

Where do atmospheric waves come from?

What ultimately drives the atmospheric circulation is the temperature difference between the equator which is warm due to direct sunlight and the poles which are cold because whatever little sun they get, especially in winter, is of low angle and there is not much solar energy reaching the surface to heat things up. This temperature difference causes a difference in atmospheric pressure that makes winds blow to try and equalize those differences.

Variations in land versus sea, the rotation of the planet, and differential heating from other causes like drought all contribute to the atmospheric developing waves that look like large swings in the pressure patterns at mid-levels in the atmosphere. Areas where the pressure is low are called troughs and areas where it is high are called ridges, like the patterns we see in topographic maps. Surface fronts are located near the boundary between troughs and ridges where the contrast in temperature and humidity is often the greatest.

Atmospheric wave pattern on February 23, 2023 showing western trough (low pressure) and eastern ridge (high pressure).

How is the current wave pattern driving these big temperature swings?

This week there is a tremendous ridge of high pressure located in the eastern US while a very deep trough is located over the western part of the country. Warm air blowing up from the south under the ridge has brought record-setting high temperatures to many parts of the East, while cold air blowing in from the north behind a strong surface low pressure center has caused winter storm and blizzard warnings all the way south to the mountains near San Diego. Record-setting low temperature and wintry conditions are occurring in those areas.

The warm temperatures in the East are of special concern for gardeners since they have caused the first leaves and blossoms to occur as much as a month early in some locations, according to the National Phenology Network. Since the average frost date for those areas is a month or two later, the likelihood of damage to fruit crops is high since freezing temperatures could significantly reduce the production of fruit in gardens and orchards if they occur in the next month or two. It could also cause damage to many other flowering plants that are fooled into blooming early due to the unusual warmth.

Freeze damage to sweet cherries. Source: Virginia Extension.

Will atmospheric wave patterns change in a warmer world?

Is the unusual warmth due to global warming? This is a hotly debated topic (if you will excuse the pun) with both proponents and skeptics weighing in. Many scientists believe that as the arctic regions warm up faster than the equator (a process known as “Arctic amplification”), the atmospheric wave patterns will become more amplified, with both deeper troughs and stronger ridges occurring. That could mean more extremes in both hot and cold weather as these waves occur. Other scientists caution, however, that the computer models used to study the large-scale wave patterns don’t always agree with global warming as a root cause. If we do have generally warmer conditions that are punctuated by significant cold outflows our gardens will have to be able to survive the increasing variability of the weather, especially in winter and spring when there is still a significant pool of cold, dry air near the poles available to flow south into mid-latitudes.

Snow on prickly pear, 7 February 2014, cogdogblog, Commons Wikimedia.

Please feel free to share how this crazy weather has impacted your own garden. Are your trees blooming early, or has snow covered everything? If you are not in the United States, what do you see going on in your own neighborhood? We are interested to know!

The nitty gritty on how water moves in plants, part 1

***Maple sap – is it in xylem or phloem? Photo courtesy of PXHere.***

It’s still too cold here in the Pacific Northwest to see much happening outside, so it seems a perfect time to write about something you can’t see anyway. That “something” is the movement of water and dissolved substances through two pathways: the xylem and the phloem. And before you roll your eyes and go watch TikTok videos, keep in mind that learning about these transport systems is critical to understanding how plants work and caring for them appropriately. To prevent brain overload, we’ll focus on how xylem works this month and tackle phloem next month.

Cross section – xylem elements are large, empty circle. Photo courtesy of Wikimedia

Functional xylem is composed of dead, lignified cells connected into a series of tubes that move water one way – from the roots to the leaves. You can think about xylem like a giant straw sucking water out of the soil and moving it into the atmosphere. You’ll find dissolved substances in xylem water, such as soil minerals and root-stored compounds including growth regulators and sugars. Since this is a one-way highway, everything in the xylem ends up at the end of the straw, which is primarily the leaves. Most of the water dissipates into the atmosphere through the stomata (a process called evapotranspiration) and the dissolved substances are left behind.

**Water movement through plants. Photo courtesy of Wi****kimedia**

I mentioned that sugars can be found in the xylem, which will confuse gardeners who correctly associate sugars moving through the phloem. That’s generally true except during late winter when some trees, most famously maples, will produce a sugary sap in the xylem. While the exact mechanism of sap production remains unclear, we know that the sugars are coming from storage sites in the trunk and require a freeze-thaw cycle to enter the one-way xylem highway.

***The temperature in the canopy of palm oasis can be much lower than the surrounding air, thanks to evaporative cooling. Photo courtesy of Flickr (Laura Hamilton)***

While many people see this process as the plant “wasting” water, it is the only way that soil minerals can reach the leaves. In the summer, evapotranspiration lowers leaf temperature through evaporative cooling. Thus, doing anything to interfere with xylem function (like using antitranspirants) will have a long-term, negative effect on plant health. Likewise, anything in the soil that’s taken up by roots may end up in the leaves – for better or worse.

Schematic view of a Casparian strip. Image courtesy of Wikimedia

Gardeners need to think about this last caveat carefully. Plant species are highly variable in their abilities to regulate what goes into the xylem and what is left behind in the root tissue. Regulation is controlled by a barrier called the Casparian strip, which is a ring of living cells that require water (and its contents) to pass through their membranes to enter the xylem. You can think of the Casparian strip as a customs office at a country’s borders: some things are allowed in, and others are forbidden. Depending on how selective this border crossing is, soil contaminants can be left behind in the roots or carried through the plant. This is why it is so very, very, important to have your vegetable garden soils tested for heavy metals and other contaminants, and to take precautions if contaminants are found.

***Arsenic is only one of many heavy metal contaminants that might be in your soils.***

Home Greenhouses III: Basic Structure Types

Over the last few months I’ve had the chance to talk about the popular topic of home greenhouses. We covered a few of the basics in my first article, then touched on some regulations that might effect the building and management of home greenhouses in some areas. In this installment we’ll talk a bit about common structures used for home greenhouse construction so you can consider which structure(s) might be right for your situation.

Types of Structures

Greenhouse structures can range from simple to the very complex. While home greenhouses tend to fall on the simple construction side, there are still varying degrees of complexity within structures. Greenhouse structures can be put into a few main categories that we’ll cover below. The key to home greenhouse success is picking the structure that works best for you, your situation, and your budget.

Lean-to structures

Lean-to structures get their name from the fact that they “lean” on something for support. Not in a literal sense, but in a sense that another structure, usually a house or maybe a storage building, provides at least one structural wall. A common lean-to setup would have a three-sided greenhouse structure attached to the side of a house or another building. This could range from a structure that is a few square feet for starting seedlings in the spring up to a full-size greenhouse attached to the side of a building. A sun-room, if equipped for starting plants, could be considered a lean-to greenhouse (at least it would at my house, since it would be primarily for plants). This type of structure can be cost effective. You reduce the amount of greenhouse surface you have to build and cover, unless you have special issues for connecting it to your home or other structures (like foundation issues, siding that is hard to attach to, etc). You do want to make sure it is done right especially if attached to your house so you reduce the chance of damage to your home.

A quick search finds many lean-to greenhouse kits available, from small to large and every size in-between.

One benefit to a lean-to is that it can use the wall it is attached to as a heat sink – the wall absorbs heat through the day and then slowly releases it at night when it is colder. If you have a large lean-to greenhouse that serves almost like a sun room you also add functional space to the house where you can enjoy the sun on warm winter days. The heat generated by an attached greenhouse can help provide warmth for your home in winter or at least provide some extra insulation. But it can also result in excessive heat gain in the summer. Lean-to greenhouses are also protected a little more structure wise – they use the building they are attached to for structural support and can often withstand weather, like high wind, a little better than free standing structures.

Free standing structures

Simply put, a free-standing structure is one that isn’t attached to another structure. It stands on its own structural framework. There are a few different options we’ll talk about here which will be further expanded in future article installments on different types of materials used.

Hoophouse greenhouse

A hoophouse greenhouse is built out of the same structure used for a high tunnel or hoophouse. It consists of framing made by bent metal pipes and is covered with polyethylene plastic sheeting. A high tunnel greenhouse would have heating, ventilation, and watering equipment added and would therefore require connection to electric, gas, and water utilities (see my previous article about what this means for a greenhouse being a taxable “permanent” structure). You may also pour a concrete slab for the floor of a hoophouse-to-greenhouse conversion, but gravel or any other floor covering is fine as long as it provides a stable surface and weed control. The “greenhouse” that I inherited from the previous homeowners was left with bare ground so they could plant it like a conservatory.

While these may not be the most attractive greenhouses, they are usually a more cost-effective option for a high-quality, efficient greenhouse. Initial construction costs will likely be lower for a comparable quality framed greenhouse though the polyethylene covering will need to be replaced every few years as it becomes more opaque and light transmission reduces.

Free-standing structures

When envisioning a greenhouse, many people envision this type of greenhouse – one with straight sides, built with a frame covered in solid material. These are definitely more attractive than hoophouse greenhouses and can add an attractive feature to your yard. The most common way for home gardeners to build a free-standing greenhouse is through purchasing a kit. There are a myriad of kits available online these days, of varying prices and qualities usually depending on the materials used in the construction. The kits can be complex but set-up is usually pretty easy to follow though you might want to factor in the cost of hiring a contractor to help with the construction on a bigger kit. Most often these kits need to be built on a concrete foundation or pad so you’ll want to consider the cost and logistics of pouring the needed support. And you’ll also want to think about the utility connections to the greenhouse.

A frame built greenhouse. You can buy kits to build structures like this yourself, or you may want to hire a contractor to help if you aren’t handy. I saw this one at a community urban farm in Trenton, NJ.

Ridge and furrow greenhouses are framed greenhouses that are built together in tandem. These structures are usually used only in large-scale commercial production, so I’ll skip those for home greenhouses.

Geothermal greenhouses

Geothermal greenhouses are gaining in popularity, mainly because people are excited about reducing the need for using electricity or gas to control the temperature. And I say reducing, because in extremely cold weather there will likely still need to be supplemental heat. These structures are definitely different than your standard greenhouse. These structures are usually sunken into the ground to take advantage of insulation by the soil and also the constant geothermal temperatures. There is also usually a high solid wall on the north side of the structure, with a slanted roof made of light-passing material that faces to the south. This allows maximum light to enter the structure and allows that light to heat the solid wall to hold and release it during the night when it is colder. There is also usually an air intake in the ground a few hundred feet away that allows for the pulling of air through a tubing system to warm or cool the air with geothermal temperatures, depending on if it is cold or hot outside.

This geothermal greenhouse project is run by the North Platte Natural Resource District in Scottsbluff, NE. Note that there is a solid wall to the right (with trees growing against it) and that the ramp leads UP to the ground level. The floor I’m standing on in this picture is about 6-8 feet below the ground surface. In addition to standard fare, this greenhouse grows citrus, pomegranates, and figs in one of the coldest parts of Nebraska.

A walpini greenhouse is a specialized/rudimentary style geothermal greenhouse that is sometimes referred to as a pit greenhouse. Instead of building a structure, it is constructed of a pit dug into the ground and covered with a transparent roof. While they sound simple, there are several drawbacks to these structures. One – they don’t do well in wet areas or areas with high water tables, for obvious reasons. Secondly, they were initially designed for use in areas near the equator with the sun almost directly overhead. To function in latitudes far north or south you almost always have to build up a wall on one side to slant the roof cover toward the sun. And even then it is unlikely that light will reach the floor of the structure where plants are growing.

Summary

There’s a lot to consider when picking the type of greenhouse structure for your home greenhouse. There’s often a trade-off between cost, quality, and aesthetics. Options range from the simple to the complex in terms of size, structure, and function. So think about the goals you have for your greenhouse and consider the many different options available to you.

Sources

https://aggie-horticulture.tamu.edu/ornamental/greenhouse-management/greenhouse-structures/

https://extension.uga.edu/publications/detail.html?number=B910&title=hobby-greenhouses