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Weekly Gardening Guide: "Frosted Tips"

May 21, 2015

This week’s tip is about FROST. I’m pretty proud of the pun in the title, but if you weren’t a teenager in the 90’s you might not be aware that “frosted tips” was the short lived hair-dye fad made famous by Justin Timberlake during his ‘NSYNC days - It's never as funny when you have to explain it. I could never pull it off anyway – my hair can’t get much frostier.

Ahem… sorry. Back to the point. Lets talk about frost. This post may be either extremely timely, or slightly late (I hope the latter) depending on the way the forecast pans out for the rest of the month. However, late May is typically the time when enthusiastic gardeners get caught scrambling to protect their summer annuals and veggies from a looming frost warning so I thought the timing was perfect.

There are several types of frost, but the type that typically impacts our gardens is called “radiation frost”. Hoarfrost is a type of radiation frost, but typically affects things that are “higher-up” such as trees, light poles, etc. In this blog we will focus on radiation frost.

Radiation frost occurs as an object close to the ground cools down rapidly due to what is known as an “inversion layer”. Normally, due to the sun’s heat bouncing off the land, the air is cooler the *higher* you go. Inversion layers are created on calm, cold nights when there is no cloud cover to retain the leftover heat from the day, and the cold and warm temperatures invert forcing the cold air down to the ground. This rapid cooling causes water vapor to freeze (going instantly from gas to solid) and settle into the lowest lying and coolest areas. This inversion layer is temporarily exaggerated by the rising sun which forces the cold air down even more which can create new or heavier frost. Though the temperature of the air may feel warmer when the sun is up, the temperature at the ground can actually get colder until the suns rays become more direct and begin to heat up the soil. I can recall many times mowing greens on golf courses that we thought the frost had burned off, only to get trapped on a low-lying green by newly settled frost pushed down by the morning sun!

*Did you know that when a meteorologist reports its recorded temperatures, they are recorded at a height of 1.2m off the ground in a special windless chamber? The temperature at the soil level can easily be 2-3*C colder!*

So why is frost so harmful to plants?

Well, its not harmful to ALL plants. In fact many food crops will actually show rapid new growth following a frost such as kale and turnips! You have heard the term “hardiness”, and you likely know that it is best for plants which have been inside all winter, or vegetables that you have germinated indoors to become “hardened off” before exposing them to the fluctuating temperatures outdoors. Hardiness is a plants ability to tolerate cold temperatures. This is because plants have a physiological response to temperature change which is related to its ability to resist intercellular freezing (ice inside the cells). They need time to build up the solutes in their cells which will help them resist freezing. These solutes act like plant anti-freeze… or PLANTI-FREEZE if you will (okay I made that one up, but Webster’s Dictionary should really consider using it). Some plants such as kale, pansies, and snapdragon are naturally quite good at resisting cell freezing and are therefore quite hardy. Their cells contain a high amount of PLANTIFREEZE. ☺

Others, including most tropicals, begonias and tomatoes are poor resisters of cell freezing and therefore highly susceptible to frost damage.

The amount of damage seen on plant tissue is directly related to the thickness and age of that tissue. Thin, delicate flower petals are the most likely to be damaged, followed by new growth, and lastly, older hardier leaf tissue. I some cases, damage is only seen when the vascular tissues are mechanically damaged while they are frozen, ie. If it is stepped on or brushed up against. This is why you have to stay off the golf course during a frost delay! Footprints, cart tires and the like will literally *snap* the frozen leaf tissue like an icicle, and in some cases can even fatally damage the crown tissue – killing the plant.

So, what can you do to prevent frost damage on your garden plants?

Well, the best protection is to never let it happen! This may mean waiting to plant tender veggies and annuals until the threat of frost has passed… but WHO WANTS TO DO THAT!? We’re gardeners, we want our flowers and we want them NOW!

So, assuming not all of your flowers are in convenient pots which can be moved indoors overnight, there are a couple of effective ways to protect your in-garden, frost sensitive plants.

Frost blanket:
Frost blankets can be quite effective against light frost. They work by insulating the heat radiating from the plants and soil and holding it close to the plant. This can keep the temperature of the foliage above freezing long enough to outlast the freezing temperatures.

While this may not be practical for large gardens, a small garden can be protected from frost using a fan. Fans work to prevent frost in 2 ways: They mix some of the warmer air above the surface into the cold air near the ground, keeping the temperature above freezing, and they also keep the water vapour moving, preventing the frost from “settling” on the plant. If you use a fan to protect a small garden, it should be mounted 2-3’ above the ground, and pointed downward.

The soil near plants retains heat. Over the course of the night, the top 5-12” will cool down and eventually balance with the ambient temperature of the air. The longer you can prolong that process, the higher the temperature at the surface will remain, thus lessening the severity of frost. Mulch acts as insulation for the soil, and thus allows heat to radiate slowly. Think of it like a thermos for the soil!

We owe most of the world’s weather systems to one important principle; water buffers temperature change much better than air or land. This means that a wet garden will stay above freezing much longer than a dry one. It may seem counter intuitive, because we think of frost as “wet”, but the process of liquid water dropping in temperature releases “latent heat”.

A better way to explain it may be to think about it like this: Regardless of the air temperature around it, so long as the water is still water and not ice it can never be below 0*C. If that water is in, on or around a plant, it will help keep that plant above freezing too! So a very effective way to protect your plants from frost is to ensure that they remain wet during the coldest parts of the night! This may mean waking up well before dawn to give them a good soaking, or if you have the luxury, programming your sprinkler system to come on just before the frost point – this is when water vapour begins to freeze, typically in the very early morning hours around 3-4am.

Use a combination:
The best protection is a combined effort of all of the above techniques, ie. Water your mulched gardens heavily before covering them with a frost blanket! Watering will warm the soil and plant surface, mulch will help retain soil heat, and a frost blanket will hold in the heat released by plants.

If you have any questions about frost protection, or recovery please don’t hesitate to email or call us. I’m approaching 1300 words here, so I’d better wrap it up!

Let's hope we don’t need to use any of this new knowledge for the rest of the spring! Bring on summer!

Thanks for reading – and Happy gardening,

- Bryan

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