Posted by

As spring continues to bring warming temperatures, home gardeners begin the tricky business of deciding when to plant what in their vegetable garden.  In the past week or so, I have started receiving questions from anxious gardeners ready to get the season started. 

While there are some reliable predictors from historic weather data, the question of when to plant outdoors is often based off a complicated equation of both past and present weather, combined with individual plant needs and availability of plant materials, to arrive at that perfect planting date.  

Some of this calculation also rests with your willingness to take a risk that an unexpected late spring frost (like we experienced last year in late May) can either send you scrambling for some frost protection or zap those tender seedling that were nicely tucked into the garden just days before.  

Despite all the uncertainty, there are some ways we can at least stack the odds in our favor based on what we know about past weather data. The Illinois State Water Survey has maps available online to communicate these data, which vary across the state.  For our area, the earliest date of the very last spring freeze is late March to early April, whereas the latest date of the final freeze of the season can be accounted for as late as the end of the May. 

So, we often communicate the median date between those two times, which falls somewhere around the end of April for central Illinois.  This date represents the point in time when half of all years have a last frost date before and half of all years have a final frost later that is later.

However, that doesn’t mean that it’s safe to plant the entire garden in April.  Some of this calculation must be made based off the particular crop to be planted and the type of plant material selected for planting. 

Cool-season vegetables thrive in the cooler temperatures of spring and fall, including many favorites such as lettuce, spinach, cabbage, beets, broccoli, kale, peas, carrots, onions and others.  Many of these crops, can be direct seeded into the garden very early in the year to get a head start on the season. In general, most cool-season crops can be started outdoors about 4 weeks prior to our last frost, which is fast approaching if you abide by the median date of the last frost.  

Direct seeding is a very cheap and easy way to establish plants in early spring.  However, the cooler soil temperatures are often less than ideal for seed germination, which requires not only ample soil moisture, but also warm temperatures. 

One tip for increasing soil temperature is to cover the soil surface with a black tarp a few weeks prior to sowing seeds.  On sunny days, the tarp will absorb heat and transfer it to the soil surface below.  On cooler nights, the tarp serves as a thin insulator to retain the heat of the day.  Select a planting day and remove the tarp the expose your nicely warmed seed bed.  Be sure to provide ample soil moisture for seed germination since your tarp may have blocked rainfall.

To avoid any issue with germination, it often pays to start cool-season seedlings indoors or purchase seedlings from a local garden center.  As actively growing plants, these seedlings won’t require the warmer temperatures needed for germination and will thrive in the cooler spring conditions.

Warm-season veggies are a different story and I find that pushing the envelope to get them in the garden early doesn’t always pay off. These plants are most productive during the warmer part of the growing season and have little, if any, cold tolerance.  The safest plan for your favorite warm-season plants, like cucumbers, tomatoes, peppers, squashes and eggplant, is to wait for the warmer temperatures of mid to late May.  However, ambitious gardeners that are willing to provide emergency frost protection can push the envelope with early May plantings.

Frost protection doesn’t have to be anything special, it just needs to take the edge away on an unusually cold night.  I’ve used everything form true gardening products, like floating row cover, to everyday items like old sheets or 5 gallon buckets on individual plants. 

Regardless of the risk you are willing to assume with early season plantings, take advantage of these nice warm days and dry soil conditions to start prepping your garden space.  The growing season is right around the corner!