Growing in Manitoba, one of the biggest challenges is always the weather. We experience harsh winters, dry summers, wet summers, flooding, late springs, early fall... all of it. It can be 30 and sunny one spring morning, and then freeze a night or two later. This can be devastating to a gardener if they don't implement a few tricks to help prepare for frost. So, before you plant your spring garden, read on!
The first and most important factor for late spring frost is your zone. I've said it before, and I'll say it again... knowing your zone, as well as expected last (spring) and first (fall) frost dates are important for anyone trying to make the most of our short seasons.
(This is from the Government of Canada Website. And is what we mean when we talk about a zone map)
Secondly, have a weather app (we have 3 or 4 running all year that we frequently check), and check it once a day. Check the overnight lows, as well as the current trend.
Knowing when to plant to avoid frost is always a little bit about luck, and a lot about experience and using the tools you have. If you check the long term forecast in Mid-May, and there are no low nights (a +3 can turn into a -1 if it's 14 days away very easily), and the spring trend seems to be getting warmer, it could be time to plant! Then again you could get an early June frost, but unfortunately you will never plant with 100% certainty unless it's July.
(A truckload of Snap Dragons we planted last year on May 12th. This year is much different)
There are plants you can seed, or plant before the risk of frost is over. Hardy annuals such as Lisianthus, Snap Dragons, and Stock are all frost tolerant. Before planting these out, you should do what's called "hardening off" meaning that anything you plan on planting soon should spend some time outside to prepare it for transplanting. Simply put your pots of trays of flowers outside a few days/nights where you know there will be no risk of frost, and start when the days are quite warm (+12-15). If it looks like you may get some very cold (below +2) nights, bring them inside. Do this for a week or two to best prepare the plants for being in a harsher climate than the warm greenhouse.
(Zinnia field. We wait till the risk of Frost is over to plant, but we direct seed a little before since it takes a few days for the seeds to pop up above the ground)
Tender annuals are much less frost resistant. We never plant any before the May Long weekend, and even then, we always check the long term forecast and make our decisions based on that. Plants like Zinnias, and celosia can freeze very easily especially when planted straight from the greenhouse. They have had no hardening-off phase, and it doesn't take much to kill them. Most plants that are direct seeded are a little hardier than things from a greenhouse because they have experienced some hardening-off simply from being in the garden.
If you have planted your garden, and a late frost is in the forecast (these tips will work with a light frost but there is very little you can do if it's going to be -5), there are a few things you can do to protect your plants. First, cover as many things as you can with fabric or frost cloth. This will be your best option. We have used everything from bed sheets and blankets to jackets and towels in the past. Obviously the thicker the blanket, the better it will protect. If you are worried about crushing your plants under the weight of the fabric, put a tomato stake or pail under the fabric to support it. Avoid covering your plants with plastic. Plastic, tarps, and pails are not insulating (unless you have an insulated tarp... then please go ahead and use that!) Your plants will freeze if the only protection you use is plastic.
(With about 1.5 Acres of flowers, it's too much to cover with fabric)
If your garden is too large to cover with fabric, your next best option is sprinklers. We have used our sprinkler system many times to avoid both late spring frost and early fall frost. Using sprinklers to protect your garden from frost can be great, but takes some knowledge. First, if your garden is very wet, you may want to avoid this method all together. Adding water to an already wet garden can be a recipe for disaster. Most plants are very susceptible to disease and bugs if too wet, so always avoid over watering your garden if you can. Second, your plants are less likely to freeze if the garden is already very wet, so you may not need the extra water anyways (if you aren't using the sprinkler method, watering your plants really well before covering them with fabric can give them added protection). Frost typically occurs right before sunrise in the early hours of the morning. In spring, when nights are shorter, this often means the duration of time below zero is not very long. The trick to using sprinklers is to turn them on right before the frost occurs. Water is obviously warmer than ice, so it coats the plant with warmth, and protects it from possible frost. I've also heard of people watering right after a frost has already occurred to prevent the cellular damage that occurs when the frost melts, but that timing is even trickier. If you turn the sprinklers on shortly before the frost sets in (when it's still +1 or even 0) and run them for maybe an hour or two (depending on how long it's below zero for), it's unlikely your plants will freeze. In order for this to work you need an accurate thermometer and a good alarm clock. Please do not turn your sprinklers on at 10pm and leave them till 8am. Your garden will drown.
Your best protection for frost will always be your brain. Don't be tempted to plant too early, look at the forecast, and know your expected frost dates. If you need to get your hands into the dirt early plant hardy annuals, root vegetables, or perennials (only if hardened-off). If an unexpected frost comes, do what you can. Cover with fabric, water, and sprinkle if necessary.
We hope it will be a long time before we drop that F word again, but you never know in our crazy climate. Good luck!!
(Stock, one of our frost tolerant favourites)
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