The past few weeks, we’ve had spectacular weather for gardening, which has really ramped up efforts across central Illinois in preparation for the growing season to come.  This early spring warmth combined with garden centers packed full of new plants creates the irresistible impulse to plant anything and everything. 

Garden beds may already be prepped and waiting, so it just seems like the time is right, especially when you consider the ever-present need to get a jump on weeds that are already thriving. However, historical weather data may paint a convincing picture otherwise, if you take some time to run the numbers.

I have received a number of questions in recent weeks about the timing for establishment of various garden plants and always refer folks to the published frost-free dates for an illustration of spring weather.  These dates are published on the Illinois State Climatologist’s webpage. 

Frost-free dates

Frost-free dates are calculated from past measurements recorded at various weather stations around the state, that spans about 30 years of time (1981-2010).  So, these dates tell us the typical day on the calendar that we can expect the last frost each year, but they are not an absolute. 

For our area, the typical date of the last frost is April 13-22, and for a hard freeze is April 5-10.  I reported dates as a range because there are numerous weather data stations around central Illinois.  Each station has a slightly different median date of the last freeze, since each location has slightly different conditions, which is why the State Climatologist’s map expresses these data as ranges, or swaths across the state map.

The dates above would imply we are in the clear for planting to the casual observer.  However, they represent the typical dates we can count on in most years, but not all.  I think we can all remember a past, late-spring cold snap and the rush to grab everything from old sheets to cardboard boxes for temporary cover to protect sensitive plants overnight.

So, you may want to hold off planting tomatoes, peppers and other cold-sensitive plants now.  Although a sheet may protect them from death, it certainly doesn’t help their productivity to be exposed to colder temperatures.  I typically recommend waiting until nighttime lows are steadily above 45, which is usually about mid-May.

However, some seedlings in garden centers right now are completely ready to go into the garden.  Most all of our hardy perennial plants can be planted now and should do just fine even if we get some overnight freezing weather.  That includes many of the herbs that are perennial in Zone 5, such as sage, thyme and oregano.

What about cold tolerant plants?

There is certainly a long list of cold-tolerant vegetables, many of which are incredibly hardy and can be planted as early as about March 25 in central Illinois.  

Onion sets are widely available right now and are a great crop to start this week.  We just planted a small patch earlier in the week in raised beds.  Among other extremely hardy crops that are readily available as seedlings are: collards, turnips, kohlrabi, spinach, kale and various other greens. 

The taste of spinach leaves is often enhanced by cold temperatures as plants react to freezing by concentrating sugars in leaves.  So, it’s a great time to plant spinach seedlings, or it can even be direct seeded with little worry.

You may also find a variety, of less hardy but still “frost tolerant” veggies available now as seedlings, including broccoli, cauliflower, beets, brussels sprouts, chard and leeks.  The recommended planting date for these crops is April 10 or later in our area, so now is the time to get them going.  All of these plants prefer the cooler spring temperatures and actually need to be started now to maximize growth and harvest before too much warm weather stops their production.