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Elisabeth C. Miller Library

Keyword: Frost

Ribbons of ice from plant stems

This winter, I was walking past a neighbor’s garden and noticed a few dried stalks (not sure what the plant was) that were sprouting a candy floss-like substance. When I got down close, I could see it was ice coming out of cracks in the stems. What causes this phenomenon? Does it happen to only certain types of plants?

 

You were fortunate enough to witness an example of frost flowers, also called ice flowers, or crystallofolia, a term coined by Bob Harms (University of Texas, Austin) to distinguish the phenomenon from “frost flowers” which are sea ice. Not all plants exhibit these fanciful formations of ice crystals, but sometimes their common names will hint at this potential. Verbesina virginica, native to most of the central and southern reaches of the eastern United States, is known by the names frostweed and white crownbeard; Helianthemum canadense is sometimes called rock frost or frostwort. This is not a widespread occurrence, and there is no clear pattern dictating which plant families or genera are likely to produce these ribbon-like excrescences. A few others which do this include American dittany (Cunila origanoides), Isodon excisa and I. rubescens.

In a column (from December 18, 2013) called The Buzz [no longer available online], Memphis Botanic Garden’s website explains the formation of flowers as follows: “When the ground is warm enough for the plants’ roots to still be active, but the air temperature drops below freezing […] juices from the plant are expelled through slits in the stems […] This may happen multiple times over the winter since our ground rarely freezes far down, but once the moisture is gone, so are the frost flowers.” The theories and explanations of why certain plants do this are far more complex. It may have to do with the xylem rays which carry sap from the center to the periphery of the stems, according to James R. Carter of Illinois State University. Plants with prominent rays are more likely to have ice flowers, which may be using the xylem rays as a source of fluid.

If you would like to increase the odds of witnessing these fascinating ice formations again in your own garden, you could try growing some of the plants on Carter’s list [no longer available online] (avoiding any which are invasive in our area!).

  • Anemone halleri
  • Ceratostigma willmottianum
  • Echinacea species
  • Eupatorium cannabinum
  • Helleborus argutifolius
  • Origanum vulgare
  • Plumbago auriculata
  • Salvia coccinea

To this list, I would add Monarda didyma, the only plant on which I have ever seen frost flowers in Seattle.

Should you wish to read more, and see additional illustrations, there is an article by James R. Carter entitled “Flowers and Ribbons of Ice” in American Scientist (September/October 2013 ). The website Kuriositas also has a page of photographs of “Frost Flowers: Nature’s Exquisite Ice Extrusion.”

This question and answer were featured in the Washington Park Arboretum Bulletin’s Winter 2016 issue, available here.

Garden Tip #46

Seattle’s average last frost date is April 20, but with signs of the garden coming to life all around us it’s tempting to ignore that mid-spring date. With a little knowledge a gardener can predict frost and take measures to temporarily protect tender plants. If the sky is clear and the wind is blowing from the northwest, get ready to take action. One simple technique is covering a tender plant with an old sheet. For more frost prediction signs and protection tips go online to A Gardener’s Guide to Frost

Camellia winter protection

How can I protect my camellia shrubs from very low temperatures?

 

The website of the International Camellia Society has a discussion of camellias and their cold tolerance, indicating that most spring-blooming Camellia japonica and fall-blooming Camellia sasanqua cultivars will survive a Washington, D.C. winter (zone 7, compared to our zone 8), but there is some concern about extreme cold and drying winds, and sudden drops in temperature (see excerpt below on winter protection, from the University of Georgia Cooperative Extension Service):

Winter Protection

Covering a camellia plant provides frost protection but does little good in a severe freeze. If plants are covered with cloth, plastic or paper, prop up the cover so that it does not touch the buds. Put the cover on after the sun goes down and remove it before mid-morning the next day.

There are additional approaches to providing winter protection against plant damage. Maintain good soil moisture, especially just before freezes. Maintain adequate nutrition by following current fertilizer recommendations. Plant in locations that provide moderate winter shade. Select varieties with good winter hardiness.

The article below from the North Carolina State University Extension has a good general discussion of winter protection for camellias.

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Rhododendron won’t bloom

This is the second year in a row that my rhododendron Blue Peter has flower buds but they are dry and somewhat dark and have no flowers at all. These buds are easy to deadhead. Can you help me salvage this rhododendron, which is very old, and beautiful when it blooms?

 

In order to get an accurate diagnosis you will need to take a sample of your plant (including both healthy and affected parts if possible) to a Master Gardener clinic. If they do not know what it is, ask them to send the sample to the pathology laboratory in Puyallup. It is best to go through Master Gardeners first so you will not be charged. If you send the sample yourself there will be a fee.

Meanwhile, several sources mention frost, drought, and “bud-blast” (unlikely in the Pacific Northwest) as potential causes of bud failure. Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought reports from various places give mixed results from applying magnesium (usually as Epsom salts)… By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently loose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.

Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244.

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on protecting plants from frost

I have some newly planted small shrubs and trees which are now in 3 inches of hard frozen snow. Everything was mulched well before it snowed. Yesterday I covered the plants with large inverted plastic pots, sheets, rice bags, and whatever I had at hand. The plants that are completely under pots will get no light. Can they stay in the dark through this entire cold snap (a week to ten days)? I’m not worried they will overheat under the pots,
with the sun so low in the sky, and the temperatures so cold, but should I be concerned with lack of light?

 

I have some personal experience with using just such pots to protect perennials, leaving the pots in place (and the plants in the dark) for a week or more. I would expect your plants to be fine. They will not overheat, though drainage holes help with air
circulation. Sheets and rice or burlap bags are fine as well, though the weight of
snow (in addition to the cold) can damage or break plants. You might want
to check the status of the covers if more snow falls. In addition, your mulching will help protect the roots. If the shrubs and trees are deciduous, they do not photosynthesize in the winter (low light and low moisture levels serve as the limiting factors, in general), so your plants will not suffer from the dark environment in
the short term. If the trees and shrubs are evergreen, they do photosynthesize in the winter, but the process is subject to the same limitations mentioned above. Again, no worries in the short term–frost can cause more damage to most plants than a few days in the dark.

Once temperatures are above freezing (32 degrees F or 0 degrees C)–and don’t forget to
consider nighttime temperatures–you can uncover the plants. Usually our
cold weather doesn’t last too long, so I think you can adopt the philosophy “better safe than sorry” and leave the pots in place until you are sure the cold weather has passed.

Below are some helpful links for additional information about predicting
frost and protecting your plants:

last frost dates

This year is my first in a house with a yard. I’m very
excited to try my hand at growing some food this year. Many of my seed packets say to plant after the last frost or when the soil reaches certain temperatures. Having never planted anything at the beginning of a season before, I
have no idea when any of these temperatures happen around here! Can someone give me some temperatures and approximate times when they are normally reached?

 

The last frost date in Seattle can be as early as
March 22, but to be on the safe side, April 15-20 would be more
definitive. This information(now archived) can also be found on the web site of local
gardening expert, Ed Hume. The web site also provides further details. Excerpt:
“The last frost date for an area is the last day in the spring that you could have a frost. The average last frost day is the date on which in half of the previous years the last frost had already occurred (so about half of the time it will not frost again and it will be safe to plant tender plants). Most planting directions are based on the average last frost date. The calendar based directions I give (Now it is time to… etc.) are usually based on an average last frost date of April 1st.

An important thing to realize about last frost dates is that the actual date of the last frost is different every year. It can be much earlier than the average or much later. This is especially important for tender plants that can be killed by a frost. For hardier plants, the average last frost date is more an indicator of general growing conditions than a danger sign.”

The closer to the water the garden is, the milder the temperatures.
The moderating effect of Puget Sound or Lake Washington, for instance,
which results in milder winter temperatures, extends inland for some
distance. If your garden is more than a mile or so from water, that
moderating influence could vary. The last frost date for Vashon Island is
April 5; for the Sea-Tac area, April 9. Again, add at least a week and
check your own garden temperatures and patterns.

tree Peonies winter protection

I have two three-year old Tree Peonies – each in a 30-inch pot. Both have buds – what can I do to protect them from the coming freeze – or will they be ok? The pots are way too heavy for me to move. They are sitting on a blacktop driveway margin. I have no dirt to bury the pots into. Do I wrap them? Would bubble wrap work?

The new one I planted Sunday is covered with an inverted pot – will that be enough?

 

Your peony (Paeonia) buds will probably be fine, but don’t take any chances!
Protect the pots with bubble wrap and cover the tops with bed sheets or some other cloth. I think the inverted pot over your new plant should be sufficient.
Tree Peonies are quite hardy. The frost would be much more damaging if it came in March or April when plants have leafed out.

Here is an article on predicting frost from Michigan State University.

University of California’s Marin Master Gardeners (now archived) also has useful tips on how to protect plants from damage by low temperatures. Tree peonies are not among the plants most liable to be harmed by the cold.

frost and bulb foliage

Do the many bulbs that are rising up to 8″ out of the ground need to be covered since the forecast has temperatures down to 17 degrees F these next few mornings?

 

The bulb foliage should be fine as it is. Any flowers, on the other hand will probably turn to mush. If you’re feeling protective or nervous you may want to cover up the foliage with burlap, cloth rags, sheets, etc. Once the clouds come back you can remove the protection.

Here is an article on predicting frost from Michigan State University.

rhododendrons and autumn frosts

A friend in Illinois has sent a photo this spring of a very healthy looking rhododendron – leaf buds fully elongated and beginning to unfurl, while the green, blunt flower buds remain unopened. The flower buds don’t look brown, diseased, frozen or injured, but they remain tightly closed, foliage bud growth preceding blooming. He says he has 6 plants doing the same this month. Possible reason?

Though we can’t diagnose plant problems by phone/email, early autumn frosts can inhibit flowering and not all buds are equally affected.

“Autumn frosts: These can lead to damage…if they either occur in early autumn or immediately after a late season warm spell. Continental climates with extremes of heat and cold are more likely to suffer sudden temperature changes than those with maritime climates…A sudden temperature drop will catch a plant before it has had a chance to reach maximum hardiness and it may suffer accordingly, even if normally perfectly able to withstand such a temperature in mid-winter…Speed of ripening varies considerably…There is also a variation in the hardiness of flower buds compared to foliage and growth buds. Commonly, flower buds may be as much as 10 F. less hardy than foliage…”
(Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 119-120)