Last frost vs Average last frost?

When I was figuring out when to start planting, I was trying to determine my last frost date. I found a few references to the Average Last Frost date of May 3rd, but when I asked at garden centres, they always told me not to plant before May 24th. When I'm planning my SFG, should I use the average last frost or what would you recommend? This year, I used the average plus 2 weeks, although we've had such a warm spring that I could have started everything much earlier.

I use the 50% chance of frost for everyhting except the plants that are totally frost intolerant (toms, basil, peppers, squash). I
googled frost date and found a calculator for that info which gives frost dates from 50-90% based on your zip code.

herblover wrote:I use the 50% chance of frost for everyhting except the plants that are totally frost intolerant (toms, basil, peppers, squash). I
googled frost date and found a calculator for that info which gives frost dates from 50-90% based on your zip code.

Herblover would you be able to post a link to this calculator?

I got my info from:

http://davesgarden.com/guides/freeze-frost-dates/

just because it showed up first on google. It gives 10% thru 90% freeze/frost dates for nearby weather stations for 24°F, 28°F, and 32°F for spring and fall if you type in your zipcode. If you want the info direct from the horse's mouth, it's kind of fun to poke around at:

http://gis.ncdc.noaa.gov/map/viewer/#app=cdo&cfg=cdo&theme=normals&layers=01&node=gis&extent=-149.3:20.2:-60.1:69.6&custom=normals

although it isn't 100% user friendly (click on "annual climate normals" to get started).

Unfortunately, it's all based on NOAA climate data, so you can't get similar info for London, Ontario. I've poked around on the web a little, and can't find similar info from Environment Canada or any other source for Canadian locations (not to say it doesn't exist, just that I can't find it). But in extremis, maybe you can get info for Port Huron, MI (zip = 48060) and Niagara Falls, NY (zip = 14301) and kind of interpolate.

barneca wrote:I got my info from:

http://davesgarden.com/guides/freeze-frost-dates/

just because it showed up first on google. It gives 10% thru 90% freeze/frost dates for nearby weather stations for 24°F, 28°F, and 32°F for spring and fall if you type in your zipcode. If you want the info direct from the horse's mouth, it's kind of fun to poke around at:

http://gis.ncdc.noaa.gov/map/viewer/#app=cdo&cfg=cdo&theme=normals&layers=01&node=gis&extent=-149.3:20.2:-60.1:69.6&custom=normals

although it isn't 100% user friendly (click on "annual climate normals" to get started).

Unfortunately, it's all based on NOAA climate data, so you can't get similar info for London, Ontario. I've poked around on the web a little, and can't find similar info from Environment Canada or any other source for Canadian locations (not to say it doesn't exist, just that I can't find it). But in extremis, maybe you can get info for Port Huron, MI (zip = 48060) and Niagara Falls, NY (zip = 14301) and kind of interpolate.

That is the one I used; I couldn't remember the name of it.

clightowler wrote:When I'm planning my SFG, should I use the average last frost or what would you recommend?

Part of your decision will be based on your aversion to risk. If you enjoy gambling, you might plant early; and if you don't, you might plant late.

I'm not a gambler; this is how I approach your question: I always plant cool crops very early. If they freeze to death, I will still have time to re-plant. I'm more conservative about the planting of warm crops. I will plan to plant them around the 50% mark, but then make last minute adjustments based on my SWAG as to what the weather will be. I'm comfortable with the 50% mark because I have a few years experience at learning how to quickly and easily provide the plants temporary protection from frosts.

Another consideration is the length of your growing season. If it's long enough, you could just be patient and wait until all danger of frost has past.

The davesgarden link is probably all you need (it is, after all, something of a crapshoot each year) but it's imperfect. For me, for example, the three stations it reported didn't include the one closest to me. Here are some quick and dirty instructions for using the NOAA site I linked to above, where you can choose which stations to use, and which also gives similar info for a wider range of temps.

1. go to http://gis.ncdc.noaa.gov/map/viewer/#app=cdo&cfg=cdo&theme=normals&layers=01&node=gis&extent=-149.3:20.2:-60.1:69.6&custom=normals

2. In the "Layers" box (click on the stack of papers icon at top left if it isn't already showing) check "Annual Climate Normals" and uncheck everything else.

3. Zoom in on the area you're interested in.

4. In the "Select Tools" box (click on the wrench/screwdriver icon at top left if it isn't already showing), use one of the options to choose how you select your stations, either by drawing boxes, or by proximity, or click on the flag to use the "Select By Location" drop down menu to get a list of stations (if using the "Select By Location" option, it's most useful to choose "by county").

5. A "Results" box will pop up. Use the checkboxes to choose the ones you want.

6. Click "Get Selected Data" at the bottom left of the "Results" box.

7. For one station at a time, choose "View Full Details".

8. In the grey box at the middle to bottom right, choose 2010 for the year (this is actually your only option) and click "View Data"

9. You should get a table based on 1981 to 2010 records, giving 10% thru 90% probabilities for spring and fall for 16°F thru 36°F, and probabilities for the length of your frost-free season.

10. You'll sometimes get a blank table, which means that weather station doesn't have sufficient data to create the table. If that happens, choose another nearby weather station.

11. If you are a true engineering nerd gardener (like me), you can copy and paste this table into an excel spreadsheet, graph it, filter it, compare it to other weather stations, etc.

Thanks for all the info and advice!

For Ontario, I got my dates from the Ontario Ministry of Agriculture and Food, from their page 'Climate Zones and Planting Dates for Vegetables in Ontario' (You'll have to Google, as newbies aren't allowed to post links for 7 days)

I guess that as a newbie I'm not familiar with which plants are hardy and which aren't, and I was afraid that frost would kill everything. I think that for next year, I'll go with the average, but I'll start enough seedlings to re-plant if the first ones don't make it. I'm not always very good at remembering to cover plants if there's a frost

I think the link you mean is:

http://www.omafra.gov.on.ca/english/crops/facts/climzoneveg.htm

Although it doesn't have the kind of data-laden tables that engineering types would love, it does have a useful discussion on probabilities of frost after certain dates, including an explanation at the bottom about when to plant frost-hardy and frost-sensitive crops (and which are which).

Just call your local extension service who will have the info for your county. At best it's a gamble anyway.

Here in Ottawa, Canada our last average frost date is May 11. 

Yikes! that means I have another 4 months and 14 days OR a total of 135 days to get ready.

Oh well, there's lots to keep me busy ... planning the garden, ordering seeds, starting seeds indoors, composting, and once the snow goes away building more garden beds. 

Our last frost date (50% probability) is Feb 3! But the soil isn't warm enough for summer crops until March or April.