Feel the Heat: Temperature and Germination

 

In most parts of the country it is time to dust off the seed starting trays, pick out your favorite seeds, and get a little plant propagation going on.  There’s definitely a lot of science (and perhaps a bit of art) to successful seed starting.  While the process starts (and relies on) the imbibition of water, one of the biggest factors that affects the success, efficiency, and speed of seed germination and propagation is temperature.  Germination relies on a number of chemical and physical reactions within the seed, and the speed and success of those reactions is highly temperature dependent. Respiration, where the seed breaks down stored carbohydrates for energy, is probably the most notable process involved that is temperature dependent (source).   Think of it in terms of a chemical reaction you might have done back in your high school or college chemistry class – there’s an optimum temperature for the reaction and any lower and higher the reaction might slow down or not happen at all.

Thinking of it this way, seeds and germination are just like Goldilocks and her porridge – there’s too hot, too cold, and “just” right.  Seeds are the same way – there’s a “just right” temperature for germination. The seeds of each species has a different optimal temperature for germination with a range of minimum and maximum temperatures for the process.

Why is important that seeds are started at their optimal temperature?

The optimal temperature is the one at which germination is the fastest. This may seem to only have consequences for impatient gardeners, but slower germination speeds increase the days to emergence for the seeds, which in turns means that the seeds and seedlings have a greater chance of failure. The early stages of germination are when seedlings are most susceptible to damping off, which can be caused by a number of fungal pathogens (Fusarium spp., Phytophthera spp., Pythium spp., etc.) that basically cause the seedling to rot at the soil level. These pathogens (as well as decomposers in some cases) can cause seeds to rot or decompose before emerging as well.  That’s why you’ll sometimes see seeds that are slow to germinate (or traditionally direct sown like corn, beans, and peas) treated with those colorful fungicides.  The fungicide gives the seed and seedling a little bit of protection (for a week or so, depending on the product), which is handy if you accidentally sow them before soil temperatures are optimal or if the species is slow to germinate.

If emergence is really slow, there’s also the possibility of stunting or failure due to exhaustion of the stored carbohydrates that the seed relies on until it begins photosynthesis.  So the closer to the optimal temperature the seed is, the faster the emergence and the highest percentage of germination success.

Image of graph showing relationship between soil temperature and seed germination.

What does this mean for home gardeners?

Whether you are starting seeds indoors or direct sowing outdoors, knowing the germination temps can help increase your likelihood of success.  You can find a variety of resources for the optimal germination temperature for your selected crops.  In general, most warm season plants, like tomatoes, peppers, and summer flowers are in the 70-80 °F range.  This is why most of the warm season crops are started indoors – so temperatures can be controlled to higher levels.

For vegetable crops, here’s a good resource for basic germination temperatures.  And here’s one for a few annual flowers.

Many of the cool season crops germinate at much lower temperatures, which means many of them can be directly sown early in the season rather than started indoors.  Crops such as spinach, lettuce, and other leafy greens have these lower germination temps and typically perform better if germinated at lower temps.

Germinating a variety of plants for our 2018 All-America Selections trials

It should be noted that this is for the soil temperature, not the air temperature. If you’re starting seeds in your home, most people don’t keep their homes in the 75 – 80 degree range in the winter.  Many commercial operations use warmed tables or beds for seed starting, rather than heating the whole facility to the necessary temp – it would be expensive.  For home growers, supplemental heat mats can help increase soil temp without having to heat a whole room.  In a pinch, you can even clean off the top of your fridge and keep seedlings there.  It is higher up in the room (heat rises) and most refrigerators create some amount of external heat as they run.

For any seeds that you’re direct sowing outdoors, whether they require higher or lower germination temperatures, you’ll have more success if you plan your sowing around soil temperatures rather than calendar dates (planting calendars can be good for estimation, though).  Investing in a soil thermometer can offer detailed information on the specific temperatures in your garden soil.  Or, if you have a good weather station nearby many of them have soil temperature probes that could give you a good idea of what the soil temperatures are in your region.

Direct-sown lettuce germinating for a fall crop

But don’t let the cool/warm season crop designation fool you – the Cole crops like cabbage and broccoli actually have an optimal germination temperature on the warmer side, but grow better in cooler temperatures to keep them from bolting (flowering).  This is why they need to be started indoors for spring planting, but you can start them outdoors (even trying direct sowing) for fall crops – they germinate in the heat and then slow growth as the temperatures drop.

Understanding mysteries of plant diseases: Diagnosis and Detection (Part 2 of 3 in this blog series)

Something is wrong?

Sometimes its subtle sometimes its not–like here with powdery mildew on coast live oak.

Do you ever have a feeling that there is something wrong with a plant? It’s just not healthy looking, or it has not grown for awhile? As we discussed in the last blog, disease is a process–it occurs over time. When in the disease time-line you notice the process, can be quite varied. Some astute gardeners may know something is wrong before there are symptoms, others may not take notice of the process until the plant is dead. Your disease detection acuity, or disease intellect, is largely dependent on your ability to recognize when the disease process is happening.  Early recognition gives you a chance to interrupt or limit the progress of disease or “control” it.  This blog is all about enhancing your disease detection acuity.  In the last of the series, I will cover what we can do about plant diseases, their prevention and control.

Sycamore anthracnose disease has vein-following necrosis as one of its foliar symptoms.

Symptoms and signs
Plants respond to challenges from a disease agent by producing symptoms. Symptoms are physiological changes in plants which we can see. Yellowing leaves, necrotic (dead) areas of leaves, stems, flowers or roots are common for many diseases. Some symptoms are very subtle. Slowed growth may be the first symptom of a systemic disease that is spreading within a plant’s vascular system or destroying its roots. Some plants can have 75% of their roots killed by pathogens without any visible symptoms on foliage. Most of the time when symptoms are this subtle, the plant is not growing at the same rate it would if it were healthy. Absence of new foliage, short internodes (distance between leaves), lack of initiation of flowers can all be symptoms of disease. Another subtle symptom is an overall color change that takes away a Plant’s “brightness” or healthy glow. I think most gardeners recognize this, but may not associate color dullness with disease. When subtle symptoms are detected, it is always a good idea to check the roots to see if they are functional (not rotted).

Overt symptoms are easy to distinguish. Fire blight is a good example—the bacterium Erwinia amylovora is spread by bees to flowers where it invades the floral nectaries. Bacteria migrate into shoots and stems and turns them pure black.

Early infections of the fire blight bacterium kill the peduncles of flowers

The disease proceeds rapidly in springtime during bloom and the symptoms (necrosis) are striking. Blights, anthracnose diseases, and canker diseases all produce necrotic tissue symptoms that are easily distinguished from healthy tissues. Even root rot is overt if you take the time to look at the roots!

Fungal hyphae of Armillaria fuse into a mycelial mat under the bark of Peruvian pepper.

Signs are the causal agents of symptoms. Fungal hyphae (collectively mycelium) growing under bark or on plant surfaces are easily observable signs. Just like symptoms, signs can be overt or cryptic. Armillaria mushrooms form in large clusters around the bases of infected trees and are easily identified, but the fruiting bodies of canker diseases (pycnidia) are very small and look like small pepper granules on the surface of a dead twig or branch. Plants often form galls (a symptom) that form around insects or bacterial pathogens (signs). Observing plants carefully to look for signs can be quite diagnostic. For instance if you observe the symptom of distorted new growth on your grape or rose and then carefully examine the tissue with a hand lens you may find the sign of mycelium from powdery mildew. Symptoms often develop after signs and many signs only form in the dead tissue or after the disease has produced much damage.

Fruiting bodies appear as black dots in this necrotic coast redwood tissue. Also note signs of white mycelium

Since signs are often reproductive structures of a pathogen, they are very helpful in pathogen identification. Many microbes have signs and cause significant disease but their signs are microscopic and thus hard

Phytophthora mycelium and chlamydospores are impossible to see in garden soil. These signs are not visible in-situ.

to observe.  The mycelium and spores of many Phytophthora spp. that cause root rot of trees, crops and flowers are invisible in-situ.  They can only be visualized by isolation in the lab.

Internet searches and labs
So you have observed symptoms, you think you have signs but are not sure. What next? There are thousands of plant diseases, and we see new diseases at an ever increasing rate as we explore growing new plants in new places.  Accurate disease diagnosis is beyond most gardeners.  Certainly you can narrow things down by looking at google images of diseases listed for the plant in question. But you can also be misled by google searches.  I would trust only University-based web pages, as there is a lot of mis-information from other sites that are inaccurate or outright incorrect in their diagnoses. In many states Cooperative Extension offices have personnel that will look at samples for you, or may refer you to a diagnostic lab that can examine or isolate the pathogen from your plant sample for a fee.

If you go to a lab for diagnosis, it can rapidly degrade into incorrect or inconclusive findings. Lab analysis, isolation and pathogen identification work well if you already have a suspicion of what your pathogen is.  You are just seeking confirmation.   Samples sent to a lab found without pathogens may not not indicate the plant was not diseased.  Samples degrade as soon as they are taken, they may not be examined right away at the lab or they may not have been transported correctly (ice chest away from sunlight). Sometimes pathogens just don’t survive well in samples, and will be hard to detect in lab settings. And most frequently, the lab usually does not get a sample with the pathogen in it.  A good example is branch die-back symptoms on a tree.  So the gardener brings in dead twigs, but the twig dieback is actually caused by extensive root rot.  The gardener never even thought to look at roots, because the twigs were the dead part. The lab of course finds no pathogens, only saprophytic organisms, which it lists and leaves the sample submitter confused and wondering if they are pathogens.   Labs are best used to confirm something you already have strong suspicions about. You have the fruiting bodies (signs) the symptoms match on-line versions of the disease you are looking at, on the same host, and everything seems right, but you want to be sure. Then a lab is useful. Especially if they get a good sample with signs present.

There are some useful test kits that home gardeners can use to confirm their diagnoses.  Lateral flow test strips are available that detect pathogen analytes. These are especially useful in testing for plant viruses and the diseases they cause.  While the cost of each test is low, there is usually a requirement to purchase a number test strips, so the cost can be over $250 to purchase a number of lateral flow test strips. The test for Phytophthora (root rot organism) is quite effective, gives results in five minutes and requires no special chemistry or long incubation periods. Some of these diagnostics are species specific, some like the Phytophthora kits, only detect the genus, not the species of Phytophthora involved. Test strips are specific to the disease at hand, so you would already need to be pretty certain of what you have if you are using these. Like a lab, they can confirm what you suspect.

Another handy way to diagnose disease is to use a host index. This is basically a list of diseases occurring on a list of different plants. Cynthia Westcott published the most important host index for ornamental plants, but it is long out of print now, and no longer published. Her plant disease handbook can still be found occasionally at Library book sales. The host index by Farr and others, “Fungi on plants and plant products in the United States” produced back in 1994 by the American Phytopathological Society is still in use by most plant pathologists because in its twelve hundred pages, you can likely find what you are looking for.

So after looking at symptoms, perhaps some testing and examining a host index, you think you have your diagnosis. So what? What can you do with a diagnosis? Well this is a jumping off point for reading the literature on a particular disease and its causal agents. Understanding the disease, its processes and timelines for disease progression will assist you in building an effective control program for your plant or garden.  At least you can decide if you dig it up and start over, or weather there is a chance of saving your plant and helping it to resist and recover from the pathogen at hand.  Astonishingly, many plants are treated (even by professionals) without an accurate diagnosis.  Know your pathogen and you will know the range of its effects on your garden plant and you can research ways to limit its damage and spread.  Next time I will talk about actions to keep garden plants healthy.

A Gardener’s Primer to Cold Hardiness, Part 2

Snowpocalypse!

Last week I discussed the mechanics of how cold hardy plants can survive temperatures far below freezing. Today we’ll consider the practical implications of this phenomenon and what, if anything, you can do to help your plants through cold snaps.

What happens when temperatures change at unusually high rates?

Remember, supercooling occurs when temperatures drop slowly, allowing water to leave living cells and freeze in the dead spaces between cells. When rates drop quickly, which can happen on sunny winter days once the sun goes down, water can freeze inside the cells before it has time to migrate into the extracellular space. When that happens, those cells die when ice crystals pierce the cell membrane. Sometimes this damage will be visible right away – you’ll see water-soaked areas in leaves, for instance, where the contents of the cells have leaked into the extracellular spaces.

Watersoaked leaf on left was frozen, while the one on the right was not.

In other cases you may not see damage until spring, especially in buds that have frozen. The scales prevent you from seeing what’s happened to the tissues in the bud, but once warmer temperatures arrive you will see brown or black leaf and flower buds. These are NOT diseased buds, though they are often colonized by opportunistic pathogens.

Partially damaged Rhododendron flower bud

What about wind chill?

The wind chill question is an interesting one. Despite the way it feels to you, wind chill does NOT lower the temperature below the ambient air temperature. It just cools things off faster than they would without the wind. For cold hardy plants, this has two important effects:

  • The rate of temperature decrease around the plant speeds up – so ice can form faster than normal. This can result in freeze damage to the plant as described above.
  • The wind itself is dehydrating, pulling away water from plant tissues and causing freeze-induced dehydration (as discussed last week). This also causes damage to susceptible tissues and is often called winter burn.

So even though the temperature itself is not lowered by wind, the rate at which it decreases and the additional dehydration stress means that plants can be damaged at temperatures they would normally survive in the absence of wind.

What can we do to help plants survive?

Before cold temperatures are expected, it is critical to mulch the soil well with a thick layer of coarse, woody mulch. This insulates the soil and roots, which are the least cold tolerant of all plant tissues. Roots never go dormant, so they are generally unable to supercool much more than a few degrees below freezing. Oh, and be sure your soil is moist (but not waterlogged). Moist soil is a better heat sink than dry soil.

Arborist wood chip mulch protects soil and roots throughout the year.

Next, be sure insulate freeze-susceptible plants. This can be done by constructing a cage of chicken wire around small trees and shrubs, filling it with leaves, and then wrapping it in burlap. Containers should be moved to the leeward side of the house or other building and grouped together. The containers need to be protected from freezing at all costs.

Heavy wet snow should be removed to avoid structural damage to woody plants.

Speaking of insulation, snow is a great insulator. But it’s not always best to leave it in place. If temperatures are cold and snow is dry and light, leave it in place to insulated tissues. But if temperatures are near freezing and the snow is wet and heavy, remove it as much as possible. Its insulative value is marginal and the damage that heavy snow can do to trees and shrubs is permanent.

 

 

A Gardener’s Primer to Cold Hardiness, Part 1

Ice crystallizing on the outside of plant tissues is often not damaging (Ralf Dolgner)

With record low temperatures in some parts of the country, gardeners are understandably worried about the ability of their perennial and woody plants to survive the cold. What today’s post will do is give you some context for understanding how plants can survive temperatures far below freezing.

Why ice floats and how this damages cells

Ice weighs less than water, but takes up more space (Wikipedia).

Everyone knows that ice floats, whether it’s an iceberg in the ocean or cubes in your favorite chilled beverage. Ice is lighter than water because its molecular structure is different: there is more space between water molecules in ice. When water freezes naturally, the molecules organize into hexagons, forming a crystalline lattice (which helps explain why snowflakes look the way they do). This hexagonal shape forces water molecules farther away from each other, resulting in a porous material that’s lighter than liquid water.

Hexagonal shapes of of ice crystals (Picryl)

As ice crystals grow, they take up more space than the water did in liquid form. You know this if you have ever left a filled can or bottle in a freezer. The pressure can blow off the lid or split the container – and the same thing happens to animal cells: the membranes are distended until they burst. But plant cells are different: there are cell walls outside the membrane which are rigid and prevent membrane rupture. However, ice crystals are sharp and can lacerate membranes, including those in plant cells.

Frozen bottles of water will either leak or explode (PxHere)

How cold hardy plants avoid freeze damage

Woody plants have evolved a mechanism to survive winters that allows ice formation in certain areas and prevents it in others. This process takes advantage of the fact that plant cells have walls, and that the area between the cells – called the extracellular space – is not alive. Extracellular space is filled with gases and liquids – including water. Water can freeze in these spaces without causing damage because there are no membranes in extracellular spaces, only cell walls. As ice freezes in these “dead” spaces, more liquid water is drawn into them by diffusion from the adjoining cells. There are two outcomes of this: one is that ice only forms in the dead space, not the cells themselves, and two is that the liquid inside the cells becomes more concentrated.

Water that is full of dissolved substances (like sugars and salts) is less able to form ice crystals because there are relatively fewer water molecules in concentrated solutions. We can see this when we add deicers to frozen walkways and roads. The ability of water to stay in liquid form at temperatures below freezing is called supercooling. Plants that are cold hardy are able to tolerate ice formation in dead tissues and avoid ice formation in living tissues by supercooling.

Salt allows water to stay in liquid form at temperatures below freezing (BU News Service)

Supercooling is different than flash freezing

We need to discard any comparison of supercooling to flash freezing, a process used for cryopreservation. Flash freezing rapidly lowers the temperature of the tissue or organism being preserved at rates far faster than what happens in nature. The water molecules don’t arrange themselves in a crystalline lattice as they freeze. Instead they form small crystals in an unstructured form, which don’t take up more space than liquid water. This means that ice doesn’t damage the cells, which are still viable once thawed.

Supercooling allows water to remain in liquid form at temperatures below freezing…but eventually everything freezes (Wikimedia)

Supercooling is a process that occurs under natural conditions, which usually mean slow decreases in temperature. This allows water to continue to move out of the cells into the extracellular space where it freezes. (There are exceptions to this naturally slow rate, and I’ll discuss those in a follow up post.)

There is a limit to supercooling

Unfortunately for plants (and gardeners) there are limits to supercooling. These limits vary with species but even the most cold hardy plants will eventually experience injury and death. The reason this happens, however, isn’t from the freezing itself, but from drought stress. Let’s look at what’s happening inside the cells during supercooling.

A schematic diagram of plant cell plasmodesmata (Wikimedia)

As water continues to diffuse into the extracellular spaces, the cell becomes less turgid; this is called freeze-induced dehydration. Without water forcing the cell membranes against the walls, the membranes start to pull away as water is lost. Eventually the membranes and plasmodesmata (which connect living cells to one another) are stretched and break. These cells are now dead – they are isolated from the rest of the plant and the torn membranes allow liquid to seep out. So cells, tissues, and entire plants that die from low temperature stress are usually killed by drought stress!

And a photomicrograph of plasmodesmata connecting plant cells (Wikimedia)

In my follow up post, I’ll discuss the practical significance of this phenomenon, including rapid temperature changes in natural and the influence of wind. And, of course, some suggestions on how to help plants survive these stressful conditions.