When is the best time to plant tomatoes

[gundogblue]

Hi all,
I live about 20 miles south of Chicago, and would like to get my tomatoes, and bell peppers planted, the 10 day weather forcast looks good, day temps in the 60's, and night temps in the high 40's and low 50's, and rain most of next week. I would like to get them planted today, but a friend told me he thinks I should wait until the end of May just to be safe from frost. So to any of you guys and gals that live in climates similer to northern Ill and plant tomatoes I would greatly appriciate your opinion on this.
Thank You,
Paul

[rainbowgardener]

Here's a chart of last frost dates:

https://usagardener.com/breaking_ground/frost_dates_usa.php

It says the average frost date for Chicago is May 17th. but I think that is the 100% last frost date... never any frost after that.

The NOAA has much more detailed charts here:

https://cdo.ncdc.noaa.gov/climatenormals/clim20supp1/states/IL.pdf

(for non Illinois US readers, just substitute the two letter abbreviation for your state where it says IL )

It gives data for two different Chicago locations. The different dates are different probability levels for frost on or after that date. So for the O'Hare area the 50% last frost date (32 degrees) is April 20 (50% of the time time there is no frost after that) and the 90% last frost date is May 10 (only 10% of the time will there be frost on or after that date).

It also gives dates for 28 degrees (killer frosts) and for 36 degrees (tender plants).

Personally I go by the 50% dates... these numbers are all based on historical averages over the last 100 years of data. In these global warming times, the historical data is no longer exactly applicable. So currently the chances of having a frost after your 50% date are really less than 50% (though NOT zero!).

[TZ -OH6]

This should give you your "safe" frost date, which is a statistical safe date rather than an average last frost date.

https://www.ncdc.noaa.gov/oa/climate/freezefrost/frostfreemaps.html

The soil is usually warmed up by then.

As for planting early and protecting from frost, there is the concept of Heat Units, which is a base growing temperature subtracted from the average daily temperature. So for tomatoes base growing temp is 50 and if the high and low for the day were 40 and 60, the tomato would have zero heat units for the day. This is complicated by the fact that in the spring time there are often proportionaly fewer "warm" hours so the high-low isn't a good way to average. Additionally you have to take into consideration the soil temperature (if it stays at 50/base temp the roots don't grow or function well) so you will probably get better growth moving potted seedlings out of the cold/into the warmth. Even if left outside in pots the pots heat up faster than the garden soil (because of air movement around them), thus promoting growth moreso than if you planted the seedlings in the ground.

https://www.gogreenninja.com/my_weblog/2008/07/knowing-how-to.html

[gundogblue]

Wow,
I'd like to thank all of you for all the great info, I book-marked all the sights you guys gave me. Acording to the charts, and the 10 day weather forcast I should be good to go if it does'nt rain, this week and all next week the weather is supposed to be high in the upper 60's, to mid 70's, and lows in the mid 50's.
Thanks Again I really Appriciate it,
Paul

[wordwiz]

As for planting early and protecting from frost, there is the concept of Heat Units, which is a base growing temperature subtracted from the average daily temperature. So for tomatoes base growing temp is 50 and if the high and low for the day were 40 and 60, the tomato would have zero heat units for the day.

But not zero growing degree days, which is a better indicator. OSU uses 50 as a base temp and 85 as the high temp cutoff, so in your example 5 GDD would be added. But OSU (at least from what I understand) does use a model that takes into consideration the number of hours above a certain threshold, rather than a simple high temp.

Mike

[rainbowgardener]

Or you can forget all the charts and plant by nature's signs.

Here's a nice article on phenological signs

https://www.ghorganics.com/Phenology.html

It says Tomatoes can be set out when lily-of-the-valley is in full bloom.


See also applestar's thread on phenological signs, here:
https://www.helpfulgardener.com/forum/viewtopic.php?p=125711#125711

Or
Plant tomatoes, early corn and peppers when dogwoods are in peak bloom or when daylilies start to bloom.
https://www.hobbyfarms.com/crops-and-gardening/garden-planning-calendars-charts.aspx

I'm not sure about the lily of the valley, I don't have any, but I know the dogwoods and daylilies haven't happened here yet. My tomatoes have been in the ground a month. Most people always tell me I plant tomatoes too early, but it works for me. They survive and set down roots even though they aren't growing a whole lot, but then when the warm up comes, they take off like crazy, while other people's are still trying to get hardened off and established. I'm the one of the few people in my area to have June tomatoes, especially without a greenhouse or hoophouse or anything...

So you will find tons of different opinions about when to plant tomatoes!

[TZ -OH6]

Here is a zip code calculator for average frost dates, safe frost dates, and growing season days.


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