It’s that time of year again. The time when spring weather is warming, we are all starting to get active in our gardens to plant things. However, lots of folks are wondering if the time is right yet. Last week’s warm weather generated quite a bit of gardening activity, and Illinois Extension offices have been filled with questions about the timing of planting.
Every spring this comes up and there is never a definitive answer because no one can accurately predict the long term weather. However, we do have statistics from past weather data to put to use which can help gardeners make educated decisions.
What are “Frost-Free” dates?
I think most of us have heard the term “frost-free date” which leads many to think there is a definitive date when no more frost can be guaranteed. However, its not that simple and I really dislike the use of the term since its misleading.
We do have good analysis of past weather data, which has been made readily available by our State Climatologist’s office. You can access these data at go.illinois.edu/FrostDates. When you start to unpack the numbers, you’ll see that many different dates and timing are available in the layers of data provided.
Accessing the relative “frost-free” state of our spring, takes some interpretation. Thankfully, all these numbers were recently updated to a more modern data set (ranging from 1991 to 2020) which more accurately portrays our current timing of late frosts.
As data from recent years have been analyzed, central Illinois gained one climatic zone, going from Zone 5b to Zone 6a. That means things are getting warmer. Our growing season has gained a few days on either end. And you can see these changes reflected in our new data on frost dates.
By reviewing data on frost dates, you can find info for Champaign County on everything from our earliest “frost-free” date (March 24) to our latest (around May 15). There are maps showing the Median date of our last frost (Apr 15) as well as the date in 1 out 10 years that we see the last frost (April 1) and the date in 9 out of 10 years we see the last frost (April 29). So, how is the average garden supposed to weigh all of this information? A lot of the answer lies in your own willingness to accept risk.
In recent days, I have had people asking about everything from the timing of direct seeding outdoors to appropriate planting time for seedlings. While each and every answer always depends on the particular plant in question, I hope to give you some idea of how to interpret all of this make decisions for your own garden.
Timing for planting seedlings
Seedlings are a little tricky to provide one answers on because, like most, it depends. As I am looking at the weather forecast for the next few weeks and thinking about April 15 as the median date for our last frost, I see pretty favorable conditions to plant most seedlings. I have quite a few that I hope to get out into the garden on Sunday, after Saturday night’s predicted lower temps.
However, I have been “hardening off” my seedlings with brief sessions outdoors the last few weeks. If you have home grown seedlings that have never been exposed to the outdoors, I don’t suggest putting them out yet. With my seedlings, I have tried to get them out for a few hours of sunshine and exposure to wind and other environmental factors every day or so. This exposure helps them prep for the real world by adjusting their growth to provide stouter stems, thicker leaf cuticles and other attributes they will need out in nature.
If you purchased seedlings that were outdoors at the nursery, they are ready to go. However, some nurseries may be selling seedings that need a bit of hardening off if they have not been exposed to the outdoors yet. Be sure to ask your nursery about this if you are unsure but seedling coming straight out of a greenhouse right now may benefit from a week or so of gradual outdoor exposure.
Timing for direct sowing
Various plants, from vegetables to annual flowers can easily be direct seeded outdoors. If you read the instructions for many of these, it may say something like “sow two weeks after the threat of frost”. So, when is that? It can be hard to interpret when the threat has passed and then pinpoint two weeks afterward.
A more direct measure is the soil temperature and that is really what this recommendation is getting at. Companies are trying to make it easier on us by giving us a timeline we can understand as opposed to telling us to measure soil temperature. However, soil temperature is also tracked and available from various monitoring stations across the state.
The Illinois State Water Survey tracks soil temperature and various other environmental parameters through their Water and Atmospheric Resources Monitoring Program, available at: https://warm.isws.illinois.edu/warm/. While this may be slightly different than a home garden space, it can certinaly help you hedge your bets. And, most urban spaces are going to be slightly wamer than monitoring sites, which are often in rural areas to reflect crop growing conditions.
For many more hardy seeds, soil temps above 50°F are adequate for germination. Crops like, kale, radish, carrots or lettuce may all be seeded at this point, which occurred about a month ago for most of central IL.
However, most annual flowers require soil temperatures of 60°F or above to germinate and grow. Looking at soil data from central IL, our closest stations are showing soil temperatures hovering around the 60°F mark. Unless we have unexpectedly cold May weather, historic data show that soil temperatures from here on will only be increasing. So now is a pretty good time to plan most annual flowers.
Some species, however, are especially warm-loving. Annual flowers like zinnias and cosmos and vegetables like cucumber and squash typically prefer soil temperature around 70°F for optimal germination. So, I think it may be best to wait for a few more weeks of warm weather before direct seeding these type of plants into the garden.
Assess your own risk
For most of my adult gardening experience, I lived in southern Illinois, which was Zone 6 until recent updates to the cold hardiness maps. Although we are now part of Zone 6 here in central Illinois, keep in mind my past as you interpret these recommendations assess your own level of comfort with risk. I have always attempted to plant early in the hopes of earlier harvest, so you may be getting advice from a risk taker. However, you now know where to find the information to make decisions for yourself based on actual data.
Ryan Pankau is Horticulture Extension Educator with University of Illinois Extension serving Champaign, Ford, Iroquois, and Vermilion Counties.