Frosts and freezes: As cold as ice…

Here in the Southeast we were surprised last week by a much earlier than usual freeze, putting an end to many gardens full of tender plants, although the cold was not deep enough to kill more cold-hardy species. In many parts of the region the frost came earlier than the 10% probability of frost indicating that early freezing conditions like this will come in fewer than one in ten years. Of course many of you in more northern interior parts of the United States have already seen your first frost this year, but here we never seem to be ready for it. In fact in parts of southwest Georgia last year’s first fall frost did not turn out to be until well into January, which caused a lot of problems for gardeners and farmers who had to deal with pests and diseases that easily overwintered the mild conditions.


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Median date of first 32 F fall frost, from

Frost versus freeze

One of the questions I often get this time of year is what is the difference between frost and freeze? The National Weather Service (NWS) puts out both frost and freeze warnings but has different criteria for each. For a frost warning, the predicted temperature may not even get down to 32 F (0 C), but may hover in the mid 30’s. For a freeze warning the predicted temperature is expected to get down to 32 F or below and for a hard or killing freeze it usually gets down to 28 F or lower. Once the area has gotten down to 28 F or lower, the NWS usually stops issuing additional freeze warnings since at that point all but the most cold-hardy plants have completed their growing season and are either dead or dormant.

A close up of a spider web

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Hoar frost on Indian rice grass. Source: NPS Photo by Neal Herbert at Arches National Park via Commons Wikimedia.

How does frost form if the temperature does not get down to freezing?

To understand frost formation and when warnings are issued it helps to know both how frost forms and how temperatures vary near the ground. Frost crystals form on surfaces that get down to freezing and have something on the surface that is conducive to seeding crystal formation. This can happen even when the air temperature is above freezing in conditions of light wind and clear skies that allow surfaces to cool to freezing temperatures by emitting heat radiation out to space at night when there is no incoming solar radiation. Conditions for this can occur with temperatures anywhere in the 30s with a reasonable amount of water vapor in the air and as long as the surface (a metal car body, an asphalt roof, or a blade of grass) can cool to the freezing point. At that point, anywhere on that surface that has an appropriate scratch, particle, or other imperfection can serve as a place for ice crystals to form and start to grow. These are called nucleation sites and allow the initial formation of an ice crystal upon which more ice can grow into delicate but visible frost.

Frost will not form if the humidity is too low because there is not enough moisture to produce visible crystals. Often frost does not damage the plants a lot because most of the frozen water is confined to the surface of the plant and does not affect the interior cell walls, although there may certainly be some damage where the ice forms. Large formations of ice crystals can sometimes form on trees or fences if the conditions are right; this is called hoar frost.

Hoar frost on ”Burgbühl” (also Hexenbühl) near Obernheim (Swabian Jura). Source: Olga Ernst, Commons Wikimedia.

Frost forecasts are also provided with the understanding that the NWS is forecasting temperature values for their thermometer heights of about 2 meters or 6 feet high, since that is how they verify the accuracy of their forecasts. In light winds and clear skies the temperature at the ground level is often colder than the temperature at the thermometer height due to cold air sinking so the ground in your garden may be colder than the forecast would predict. Frost is also more likely to form on elevated surfaces that don’t have contact with the ground, since soil temperature keeps the ground surface warmer in Fall than later in the year due to residual heat from the summer warmth. Blueberry farmers that I work with tell me that you can sometimes see quite a difference in frost damage to their bushes from top to bottom due to the different temperatures that the plant may experience at different heights above the ground. Bridges often have signs that they freeze first for the same reason.

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Saucer magnolia with freeze damage. Source: Famartin, Commons Wikimedia.

Freeze damage to garden plants

The NWS issues freeze forecasts when the temperature is expected to get down to or below 32 F. The damage that the freeze does to plants depends on how long the temperature drops below freezing and how susceptible the plant is to cold temperatures. If the temperature barely gets down to 32 F for a short period damage is likely to be minimal since the water inside the plant cells did not have sufficient time to freeze. But if it lasts longer the water in the cells freezes and, as you undoubtedly know, ice expands and breaks the cell walls causing irreversible damage to plant leaves and stems that leads to their death. John Porter provided a useful table of how different garden vegetables respond to cold temperatures in his 2020 blog on spring frosts, which underlines why some vegetables like spinach and cauliflower do better as late-season vegetables than tomatoes and melons.

The discerning reader who looks at John’s article will also note some differences between the first frost map he published in his blog and the map above, because they cover different time periods. John used the map for 1980-81 to 2009-2010, since that was the current one at the time of his post. The map here uses the 1990-91 to 2019-2020 period since the normal temperatures have been updated since John’s blog was published. Average frost dates change over time as you can see especially in some areas like eastern Oregon and northern New York State and generally, as the earth gets warmer, the first frost of fall is occurring later in the year than it did in the past (although there are a few exceptions such as parts of northern Georgia).

Frosted Kale. Source: Tracy from North Brookfield, Massachusetts, USA, Commons Wikimedia

With winter on the way, we are sure to see many more examples of frosts and freezes in the coming weeks for almost everyone other than those who live in tropical areas. For those of us who enjoy chilly weather, the magic of frosts and freezes is something we look forward to as it paints our dying gardens in icy white.

Understanding how weird weather affects our plants

Nutrient deficiency? Or something else?

I’d intended to write the column earlier in the year, but it’s as relevant now as it was in the spring. This post will familiarize you with how unseasonable weather can affect your plants. Though I’ll be focusing on my own location in Tacoma, the phenomena are global. You just have to pay attention to what happened last week, last month, last year in your own location.

Our two potted Japanese maples

Our spring started out wet and cool, which is nothing new. But it was REALLY wet and REALLY cool compared to normal. This meant that our trees and shrubs had plenty of water to fill their expanding leaves and blossoms – but the lower than normal temperatures affected leaf growth. These dwarf Japanese maples had lots of leaves, but they were tiny! And they stayed that way, because once the leaves begin to lay down cuticle, they don’t expand any longer, even when it gets warmer. These maples put on a second flush of growth in the summer. Look at the difference in leaf size, determined solely by ambient temperature.

We had an abundance of flowers on our fruit trees – so dense that I put off pruning some of our heritage apple trees so we could get an even bigger crop (our black Angus love apples). But summer rolled around and…virtually no apples on ANY of the trees. What happened?

Normally, our apple blossoms are opening when there is lots of insect activity

Well, that cool spring ensured that most of our pollinators were late to emerge from overwintering. I had wondered about them in the spring, as I could only see a few pollinating flies and no bees. But sure enough, we had almost zero pollination. No apples this year. Next year if the weather is similar I’m going to try using my battery-powered leaf blower to pollinate these trees. I’ll take pictures.

This chart only goes through August 28. We had no rain until October 20th.

Fast forward to summer – for us, a record-breaking drought (again). Our temperatures weren’t as high as last year, but we still had very hot weather and no rain. For our landscape it’s not a problem, as we have well water for irrigation. But those gardeners who have little or no supplemental irrigation may very well find that their woody plants and perennials don’t perform very well next year: perhaps fewer flowers or branch dieback will appear. This is due to root dieback that happened all summer in unirrigated conditions. The damage is only seen in the following spring, when there aren’t enough roots to supply water to emerging buds.

Crown dieback from water-stressed roots.

Being able to predict the impact of specific weather events on your landscape plants is key to avoiding misdiagnosis and subsequent misuse pesticides or fertilizer in a futile attempt to rescue them.

Oh, and if you are wondering about the photo at the top, you’ll have to look at a post from 2009 to see what’s going on.

Tulips for the desert?

Spring bulb planting time is on across North America!  Many types of bulbs do well in desert and xeric gardens: hyacinths, ranunculus, iris, narcissus, crocus, alliums can all be happy. One bulb that’s often left off the list are tulips. Why is that?

The tall flashy hybrid or Darwinian tulips that fill the catalog photos are usually considered an annual in most desert gardens. They require more chilling than the our desert winters can usually provide and can be a little fussy about soil and water.
But tulips can be very happy in xeriscapes. In fact they can get so happy they’ll set seed and naturalize in the right conditions. And which tulips are those you ask? (Yes you did, I heard you.)
They’re species tulips and are non-hybridized. They’re more of “wild” type of plant. What’s so special about them?
They’re tough, amazingly tough.

Tulips are originally from mountainous areas of the Mediterranean, Middle East and Asia. Some are also native to Southern Europe, North Africa, and several Mediterranean islands. They’re frequently found clinging to barren mountain ledges, rocky areas exposed to wind and cold, and drought ridden slopes.

Map by Tulips in the Wild
For an interactive format with species information go here:

Species tulips are shorter and smaller than hybrids but what they lack in stature they more than make up for in resilience and showy display. They’re an early bloomer in the desert southwest which is wonderful for the pollinators that are often active on warm winter and early spring days.

Their foliage is usually more varied than hybrids; some have broad, curled edged leaves, some are tall and narrow. The color varies too, from a bluish tint to shades of green. Some varieties even have multiple blooms per stem.

Many species are attractive whether the blooms are closed or opened. They often have very different coloration inside and out.

…and open

Some have contrasting pollen color which adds great visual interest.

These have a deep purple pollen.

Species tulips are usually perennial in warm winter climates. They increase via bulb offsets while many will set seed. They aren’t fussy about soil as long as it’s well drained. But like all plants they do require water during bloom and while the leaves are green but still, not as much as other bulbs. They’re perfect for xeriscaped or low water landscapes since they require less overall water than other bulbs. Plus, they prefer to be dry during their dormant season, which is summer to fall.

These tough little beauties can occasionally be found in garden centers but for the best selection shop online, search for “species tulips.” Do some homework first, and become familiar with the the available varieties.

Plant these tulips from fall to early winter. Provide full sun and good drainage, rocky or lean sandy soils are ideal and most closely approximate their native conditions.

Don’t overplant with species that require a lot of summer water. Mix these bulbs with other plants that prefer hot dry conditions. Tuck them into those corners that get spring – early summer sunshine, spots that don’t get much summer water, or put in containers that you can enjoy and then ignore during the summer. Pot them in cactus mix combined with a small amount of regular potting soil and top with an organic mulch. Remember, drainage is a must and overly rich or high organic matter conditions aren’t to their liking.

If you live in a dry or desert region and have never tried species tulips, why not give them chance. You might just discover a new favorite.