Black Frost

I love standing at our market stall in Ryrie Park. It’s a great social outing and the exchange of information is fantastic. I had never heard of a black frost until I set up our stall last Saturday. Then tale after tale of black frost was delivered directly to me. Each tale compounding learnings from the last. I love school days.

Early Thursday morning I visited my garden to see what the clear, cold night had delivered. To my surprise there was no frost evident but the leaves on some plants had turned black. They still held the form of leaves at 6:10am. By 6:30am it got colder and just as the sun hit my garden the leaves melted.

Hoar Frost

Hoar frost, is when water from the surrounding area, freezes to form a white crystally frost. But what happens when there is no moisture in the area? No frost can form so a black frost freezes the water inside the plant.

Think of the cells of the plant as a bunch of water balloons squashed together. When frozen, the balloons can be popped but the formation remains. Then sun shines on the plants and they wake up and start to grow. At this point the cells start to collapse. When defrosting in sunshine the plant structure becomes moosh. 

When hoar frost covers a plant, the conversion from water to frost creates a little heat offering some buffering from the cold. This is why a late, light frost wont hurt your plants like a hard frost lasting all night.

Beat the Black Frost

An interesting event is when the morning dew is heavy, and then it freezes just before sunrise, creating clear frozen water on plants and flowers. I have seen this many times on my hill. This clear ice encapsulates the leaves and blossoms releasing a little heat in the freezing exchange, but more importantly the clear ice provides insulation, limiting the temp to zero degrees inside the ice. 

This technique has been used to protect vineyards, orchards and market gardens for a long time, but I have never heard of it. Ive heard of poor souls spending their predawn hours running around orchards keeping fire bins alight to keep the frost off, but spraying water seems counter intuitive.

Then a new friend arrived at the table and revealed that he had a system where the frost alarm goes off at 0C and then the sprinklers come on until the threat is over. I was astonished. 1) because it seemed like a very elaborate system to have in a backyard but mostly 2) because after years of enquiry, I had not encountered this technique.  

A quick visit with Dr Google and it’s a thing, a very popular thing. Ive heard anecdotes about watering your plants at night to protect from black frost but had no success. This appears to be where that urban myth started. Like most myths, they need context and correct interpretation to use them as they were intended, or they become misinformation. 

The only areas in my garden affected by the freeze were the areas without tree cover or dense plantings. Even 6ft saplings helped deflect the cold. Lots melted, but there were many surprises. Ive been brave this winter/spring, resisting the urge to box my tomatoes and other delicate veggies, and the learnings have been grand. 

Survivors

Resprouting basil from the last frost got melted in the same pattern. Spinach is a great frost companion, onions not so much. There are tiny basil leaves, left underneath melted ones still growing.

Corn survived amongst the broad beans. Pumpkins survived under fruit tree saplings, but one in the open got mostly melted. It has since produced flowers and is starting more leaf so on they grow. It is next to a tomato that didn’t get melted. The tomato is surrounded by brassicas and snow peas but is sticking above their leaf protection.

Watermelon planted before the last frost has had mixed results, as it didn’t get much time in the ground before the cold snap. Some melted without protection, and those under drip lines or planted tightly survived just fine.

The real surprise was the potato patch. It takes a low of -5C to melt a potato top. Again, only melting the ones planted to the edge with few companions. Theres a pattern forming here. Plant densely and surround frost delicates with hardies. No need for artificial protection, autumn/winter plantings can create places for spring. 

Tomatoes

My tomatoes have been in the ground since mid September (led by my soil temp). They have survived multiple frosts, hail, sleet and now a black frost. Yes, they survived the black frost. All of them, including the ones in pots we sold at the markets.

A Kunzea hedge does a great job shielding potted delicates from the frost, as do all the hardy vegetables I planted in winter. Only the tomatoes in the centre of the garden, without saplings, got affected. It was truly amazing how these 4 tomatoes behaved.

Ive often said that BST Wildgrown Tomatoes are reliable down to -1C, -2C maybe not. Melting potatoes at -5C should surely melt unprotected tomatoes…

The four tomatoes affected were over 30cm tall, were staunch trunked, had flowers and fruit and were acclimatised to our garden. They got burnt by the Black Frost but only the tops. The leaves on top burnt black and shrivelled. The potatoes 1m away melted fully. The rest of the tomatoes with their companions didn’t notice. One of the melted tomatoes shielded a newly planted watermelon perfectly. My backyard ecosystem is in development and is already showing its worth.

Takeaways from the Black Frost

Vegetables are tougher than we think and when they have crew, they are stronger. 

Choosing Wildgrown stock and collecting seed for next season is key. 

Acclimatising your plants is super important, so get them in early if soil temp allow. 

Don’t fear spring frosts, they are an awesome part of growing in our unpredictable highland climate. The frost and unpredictable cycles are not going anywhere, so lets learn to groove with it and enjoy abundance at home. 

And remember, every day is a school day,

Stay Awesome.

The Gordon Gnohm