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We all have some idea of what the last frost date is in our area. Over time a lot of data has been accumulated, because gardeners have paid a great deal of attention to the last frost date. Numerous lists will tell you what that date is for your area. However, these dates are based on averages and only give you an general idea of what to expect. The last frost date not only varies from year to year, but with global warming it is estimated to have advanced from 5 to 10 days over the last 50 years. So how are we to decide when to move our tomato or begonia plants outside? I think it is best to follow nature's calendar.
By nature's calendar I mean looking at the plants around you, especially at the shrubs and trees to see when they are leafing out. A tree starts to put out leaves when its genetic program decides there won't be any more frost. However, all trees do not come up with that date the same way, and some are more reliable than others at figuring it out.
Trees in eastern North America follow different approaches.
Many local native trees, for instance, the sugar maple (Acer saccharum) and the quaking aspen (Populus tremuloides), the most widely distributed tree in North America, are not only concerned with how warm it is before they will think of leafing out. They also require a number of cold days. Once they have had the required cool period, they will then wait till a number of warm days has elapse before they let their buds break out. Some, like the American beech, take into account the length of daylight! These three natives are what I would call the "prudent" trees.
On the other hand, some trees, like the native birch, only respond to warmth. This is also the case of most ornamental shrubs from warmer climates, which includes lilacs (extremely cold resistant but originally from warmer climates than ours). These are what we could call the "gambling" trees. Their reward is getting a few more weeks of photosynthesis than the more "prudent" trees and shrubs. However they run a higher risk of being zapped by a late frost.
This means that a reliable way to figure out when all danger of frost is over in your garden, in a given year, is to check when the local "prudent" trees (the sugar maples and beeches in our climate or the equivalent in yours), the ones that do not respond only to warmth, put out their leaves. Exotics are more liable to make mistakes.
However there are trees that could be described as "too prudent". We have lots of native ash trees round us here and a few planted catalpas close to the garden. These usually venture to put out leaves quite a few weeks after the last frost. They are not your best indicators because they are too prudent.
It is not as complex as it might seem. In fact, trees are amazingly good at reading nature's calendar. How often have native trees near your garden got their leaves burned by a late frost?
So when do you transplant your tomatoes out in the garden? Well, if you are ready to take more risks, put them out when birches are leafing out. Otherwise, check the sugar maples. Finally, if you don't want to run any risk at all, wait for the large leaves of the catalpa to unfold.
We all have some idea of what the last frost date is in our area. Over time a lot of data has been accumulated, because gardeners have paid a great deal of attention to the last frost date. Numerous lists will tell you what that date is for your area. However, these dates are based on averages and only give you an general idea of what to expect. The last frost date not only varies from year to year, but with global warming it is estimated to have advanced from 5 to 10 days over the last 50 years. So how are we to decide when to move our tomato or begonia plants outside? I think it is best to follow nature's calendar.
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| When can you move tender plants out in the Spring? |
By nature's calendar I mean looking at the plants around you, especially at the shrubs and trees to see when they are leafing out. A tree starts to put out leaves when its genetic program decides there won't be any more frost. However, all trees do not come up with that date the same way, and some are more reliable than others at figuring it out.
 |
| The leafing out of trees around your garden is the best indicator of when the last frost will be |
Trees in eastern North America follow different approaches.
Many local native trees, for instance, the sugar maple (Acer saccharum) and the quaking aspen (Populus tremuloides), the most widely distributed tree in North America, are not only concerned with how warm it is before they will think of leafing out. They also require a number of cold days. Once they have had the required cool period, they will then wait till a number of warm days has elapse before they let their buds break out. Some, like the American beech, take into account the length of daylight! These three natives are what I would call the "prudent" trees.
On the other hand, some trees, like the native birch, only respond to warmth. This is also the case of most ornamental shrubs from warmer climates, which includes lilacs (extremely cold resistant but originally from warmer climates than ours). These are what we could call the "gambling" trees. Their reward is getting a few more weeks of photosynthesis than the more "prudent" trees and shrubs. However they run a higher risk of being zapped by a late frost.
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| Catalpa in bloom |
This means that a reliable way to figure out when all danger of frost is over in your garden, in a given year, is to check when the local "prudent" trees (the sugar maples and beeches in our climate or the equivalent in yours), the ones that do not respond only to warmth, put out their leaves. Exotics are more liable to make mistakes.
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| If you start zinnias indoor, you have to guess when the last frost will be before you plant them out |
However there are trees that could be described as "too prudent". We have lots of native ash trees round us here and a few planted catalpas close to the garden. These usually venture to put out leaves quite a few weeks after the last frost. They are not your best indicators because they are too prudent.
 |
| These tall ash trees outside the garden are not the best indicators of the last frost date |
It is not as complex as it might seem. In fact, trees are amazingly good at reading nature's calendar. How often have native trees near your garden got their leaves burned by a late frost?
So when do you transplant your tomatoes out in the garden? Well, if you are ready to take more risks, put them out when birches are leafing out. Otherwise, check the sugar maples. Finally, if you don't want to run any risk at all, wait for the large leaves of the catalpa to unfold.
For me, it's so long away that I can't even think about it. I put things out June 1. I watch the forecast and if it's to be clear and cool stuff goes in around June 7.
ReplyDeleteIt reminds me of when I was young in eastern Québec. The last frost could be in early June.
DeleteGreat information, thanks! Timing for my plants depends on the plant. Lettuce seeds go out in early May. Hanging baskets can go out then, too, because I can always lift them into the garage if I need to. Some perennials go in the ground earlier. I avoid putting out the Tomatoes until Memorial Day, because they need heat anyway. But my last frost, technically, is mid-May.
ReplyDeleteThat is about the same time table we follow here.
DeleteThat's really interesting!
ReplyDeleteGlad you found it interesting.
DeleteWhat an interesting post.My dad used to say the latest recorded frost was May 15th here; however I find closer to June 1st is better. Of course, it's been thirty five or six years since he told me to wait until May 16th.
ReplyDeleteIn terms of climate, I suppose 35 years is nothing, even if with climate change things are moving faster.
DeleteWe have an old saying that goes "Cast not a clout 'til may is out" which basically means keep dressing up warmly until the may blossom (hawthorn) is flowering which is similarly taking a lead from nature.
ReplyDeleteIt also exists in French "En avril, ne te découvre pas d'un fil" meaning the same thing. What is implied I suppose is don't get too excited by a nice days in April.
DeleteInteresting post, I must watch my trees more closely! Usually we don't get a frost after May 15th, last year though our last frost was mid April so I could have put plants out a lot earlier.
ReplyDeleteIt must be difficult to decide when to plant things out when there can be so much difference from year to year in the last frost date.
DeleteGreat post Alain! Watching native trees has never occurred to me, but it makes perfect sense. Thanks for letting me know about the euphorbia in my post. I feel reassured it will make it through the winter knowing it has done well in your garden.
ReplyDeleteGood luck with the euphorbia. The snow cover is good this winter. It should help.
DeleteIt's for this reason I am joining in with the Tree following meme this year, it will be the first time I have paid much attention to trees leafing out and although I am only following one tree on my blog, I am taking notes on the others.
ReplyDeleteI am in the 'dinnae cast a cloot til May is oot' catagory like Sue - expect we have a different accent ;)
It is a pity we cannot record the comments - it would be very interesting to hear the different accents!
DeleteThe tree meme seem very interesting.
I have read when the spring peepers sing 3 times, then it's safe for putting out the dearlings. Not too sure about that, but I'm going to be more aware this spring.
ReplyDeleteThank you for stopping Cheryl. I think the spring peepers must be a good indicatorsince they manage to sing during the coldest period (at night). Anyhow you would not miss them as they make quite a racket!
DeleteWhat an interesting post, trees are a good indicator though, our cherry trees are the first out but I always wait for the beech in may. A couple of years ago our spring was actually 2 months late and the trees didn't leaf out until early may, it was most odd.xxx
ReplyDeleteIt is interesting how there are individuality among trees. I have noticed for instance that among the same specie (maple) the first ones the turn in the fall are always the same and so are the last ones.
DeleteOur old saying is "the oak before the ash we are in for a splash, the ash before the oak we are in for a soak'' I think this reflects how different stimuli effect leaf emergence. Both these trees are late - we expect frosts here in York up to first week in June. Trouble is the last frost might be preceded by a month of no frost!. Its not a sharp cut off from Winter like in some parts of continental Europe.
ReplyDeleteAs you probably know, here the climate is very continental. We go from winter to summer with hardly any spring at all. I suppose it is the silver lining of having such a long winter. By the time of the last frost date, it has not been warm for very long. There is not much back and forth.
DeleteI like your old saying. Both oak and ash are also among the last to leaf out here. We have a lot of ash trees and they are just about to all disappear because of the emeral ash borer. They have already all died in much of southern Ontario.
Very interesting post. One of the common names for Amelanchier, Shadblow, derives from how they would bloom when the shad ran in the rivers.
ReplyDeleteThese various connections between plants and people or animals are always interesting.
DeleteI think ash trees and oaks are good indicators, as their leaves are as tender as most tender annuals/crops. In our climate they do get mistaken too sometimes, loosing all leaves to night frost. I don't want to risk with my tender ones though! So its quite tricky.
ReplyDeleteAs far with tomatoes I only grow them in my greenhouse because rain tends to ruin all the crop. It is surely time to sow them now!
I suppose Estonia has a maritime climate if you grow tomato in greenhouses. I would have thought it was more continental, but of course you are surrounded by the Baltic.
DeleteAlain hello there !
ReplyDeleteI have never heard of this before .. I was always looking at the formal stats .. but this does indeed make more sense .. I love the lilac "gamblers" .. that made me grin : )
I will be watching now as well .. thank you !
Joy
Lilac is fascinating in that it is supposed to grow best in our climate which gets very cold but it is related to the olive and comes from the Balkan.
DeleteEarly May is when I feel safe putting out my most tender plants. But we've had frosts up to mid-May some years so I always keep a close eye on the weather forecasts. But maybe I should keep a closer eye on my trees. I think they're all leafed out by then.
ReplyDeleteIt is funny that there is so much difference in out climate, but actually not a great deal of difference between the last frost dates. I would have thought your last frost date was a fair bit earlier.
ReplyDeleteI agree with Tistou, we live very close, by Baltic sea and the climate is maritime as well. I often see forecast and sometimes I note the nature calendar too, Alain. Here we say: until first birch leaf appears may not plant any flowers or veggies in soil, as I understand until mid-May.
ReplyDeleteI would think the first birch leaf is as early as you can make it.
DeleteI never thought of looking to the trees to help me gauge when it was safe to plant out tender stuff . It is usually towards the end of May depending on the weather, and I know from bitter experience that it is not worth taking a chance ! The greenhouse can be bulging at the seams but I stand firm !!
ReplyDeleteI must confess that I sometime put out a few things too early. Then on cold evenings I am out madly covering things with old blankets.
DeleteGlad to have stumbled upon your site via Rick from Cheshire. We used to live in Aberdeen Scotland where those new to gardening would constantly get caught out with planting out Summer bedding plants far too early, I blame the garden centres. First week in June was relatively safe. As for trees, the Rowan Cashmerana was always the first to open its leaves in early April, didn't even mind when the frost returned, however the native variety was always a few weeks later. Thoroughly enjoyed your post. Alistair
ReplyDeleteThank you for stopping by Alistair and for you interesting comments.
ReplyDeleteI expect that getting caught planting too early is in fact less common here as warm temperatures arrive so late. I think you are right about garden centres. The do the same thing here. One problem here is that it is engraved in stone in people's mind that all planting and seeding is done on the last week-end of May (a long week-end celebrating the birth of Queen Victoria! Even in the UK you don't have a long week-end at that date if I remember well.)
Thats quite amusing Alain, I guess you could have a prince Albert day to remind folks when to lift the Summer bedding.
ReplyDeleteWhat an interesting post. I never thought of checking trees to decide when to put my tender plants out but it makes sense. Usually it is reasonably safe by the middle of May here, but there is always some risk.
ReplyDelete