Climate change adds complexity to predicting frost-free dates by influencing weather patterns and subsequently the growing seasons across the globe. The USDA Hardiness Zones (also known as climate zones) were widely used to provide an average expectation of minimum winter temperatures. But there were never any specific frost dates.
Frost-free date prediction is getting harder
The U.S. has 13 “climate zones.” Find your climate zone in the official USDA Map of U.S. Climate Zones. The higher the zone number, the warmer the climate. To learn more about how climate zones work and how to use planting calendars go to our blog post Know Your Climate Zone.
Greater Chicago area and most of northern Illinois are in climate zones 5b and 6a. The city of Chicago and a few bordering towns are in zone 6a which is warmed by the “Lake Effect.” The collar counties are in zone 5b.
Oak Park is officially in Zone 6a. River Forest and the western suburbs are in Zone 5b. We want as little cold weather risk for our beginner gardeners as for their warm weather veggies like tomatoes and peppers.
Beginners use zone 5b planting calendar. We recommend that all Greater Chicago area beginning and intermediate gardeners follow our Planting Calendar for Zone 5b since there is less risk of unexpected cold weather. Beginners should plant at the warmer part of a planting period in our planting calendars. Still it is important to learn the tools to protect your garden from unexpected cold temperatures and heat waves.
Experienced gardeners in zone 6a (city of Chicago) can take advantage of the longer frost-free growing season. Use our zone 5a planting calendar as a reference point. The spring zone 6a planting dates are 2 weeks earlier and the fall 6a planting dates are 2 weeks later than 5b.
Average frost-free dates for spring and fall in zones 5b & 6a:
Climate Zone 5b: Last spring frost is between mid-May and late-May. The first fall frost is between late-September and mid-October. The growing season is approximately 150-180 days.
Climate Zone 6a: Last spring frost is between early-May and mid-May. The first fall frost is between early October and mid-October. The growing season is approximately 160-200 days.
Average frost date ranges vary across the U.S. The front-free date ranges for zones 5b and 6a can vary significantly across the U.S. due to regional climate differences such as altitude, proximity to large bodies of water, and other factors. Therefore, it's essential to refer to local university extension services. botanic gardens or weather stations for more precise data.
Microclimates can cause temperature variations. Be aware that even within your specific zone, microclimates can cause variations in temperature. Garden placement, such as near a south-facing wall or in a low-lying area, can cause warmer or cooler spots, respectively. Monitoring your specific conditions will give you the best understanding of your garden's climate over time.
We use the Plant Information Service at the Chicago Botanic Garden for advice on local gardening problems including planting times and weather.
How to track temperatures effectively
Use a reliable outdoor thermometer: This can help you monitor the current temperature in your garden.
Consider a digital weather station: These can provide much more accurate and real-time data, including temperature, humidity, wind speed, and rainfall.
Use online resources: Websites like The Weather Channel, AccuWeather or National Weather Service (NOAA) provide comprehensive weather forecasts, including nighttime lows.
Mobile Apps: There are numerous gardening and weather apps that can help monitor your local conditions and even send frost alerts.
Frost Protection and Soil Temperature
Predicting frost-free dates is becoming more challenging due to changing weather patterns during climate change. Therefore, be prepared to use frost protection techniques.
Soil temperature is crucial for planting food garden seeds and transplants, as it affects seed germination, root development, and overall plant growth. Different plants have specific temperature requirements for optimal growth, so it's essential for gardeners to consider both soil temperature and frost-free dates. See our Veggie Temperature Tolerances blog post for details about crops we include in our planting calendar.
For beginner gardeners, it's wise to balance soil temperature and frost-free dates. Choosing the right time to plant can minimize the risk of frost damage and ensure a successful garden. Beginners should plant their warm weather seedlings at the warmer end of the recommended planting period in spring. See our Planting Calendar for Climate Zone 5b.
Low temperatures can damage or kill warm weather crops. Monitor weather and temperature daily and weekly. Know the temperature tolerance of your plants. The night time temperatures are usually the lowest. Some plants are more sensitive to temperature than others. The duration of exposure to a low temperature is also a factor. Cold tolerance plants have varying temperature sensitivities as well. Some warm weather crops can be damaged by temperatures in the low 40s, and 50s.
How to protect your garden from frost
Check YouTube for videos about how to make the protections below.
Don’t plant outdoors until the risk of cold damage is gone. Know the temperature tolerance of each plant, follow the planting calendar and monitor the weather.
Re-plant your tiny warm weather seedlings like tomatoes, peppers and basil into larger pots (4-5” diameter, 6-7” tall) until safe planting time at the end of May arrives. Deep Roots has lot of pots you can use and keep for future years. Fill the pots with half compost and half worm castings. This way you can protect the seedlings with sufficient space and nutrients.
Bring re-potted warm weather seedlings indoors (house, garage or shed) overnight or even all day near a large window.
Setup temporary a mini-hoop house covered with heavy fabric if you have too many seedling pots to move indoors on cold nights. Use sheets, light blankets, frost cloth (frost shield) and canvas supports by hoops. Place the hoops in a raised bed or in the ground. Remove the cloth during the day if temperature permits. The frost cloth is best and is sold at many hardware stores and big box stores. Use metal fencing or 1/2” inch PVC pipes for the hoops. Place stones and boards around the bottom of the fabric to keep out cold and keep it from blowing away. Make sure the fabric doesn’t touch the plant foliage. Don’t cover with plastic or tarps which will collect moisture and harm your plants.
Floating row covers: Floating row covers or frost blankets can be draped over your plants to shield them from small dips in temperatures. They allow light and water to pass through while providing insulation. But this often can’t raise the temperature adequately to protect from low temperatures..
Cloches: Cloches are small, transparent covers, often made from plastic or glass, that protect individual plants. They create a mini-greenhouse effect, trapping warmth and moisture around the plant. Use them at night and take them off in the morning.
Cold frames: Cold frames are bottomless boxes with a transparent lid, usually made of glass or plastic. They can be placed over garden beds to provide insulation and protect plants from frost.
Greenhouses: Greenhouses, even small or portable ones, can provide a controlled environment to protect plants from frost and extend the growing season. Small inexpensive fold-up green houses can be found online.
Water-filled containers placed in a hoop house. Placing water-filled containers, like plastic bottles, next to your plants can help retain heat. The water absorbs heat during the day and releases it slowly at night, creating a warmer microclimate.
Mulch: Adding a layer of mulch around your plants can help insulate the soil, maintaining a more consistent temperature and protecting the root system from frost.
“Grow Your Own Food” blog posts
See the full list of our Grow Your Own Food blog posts. Each post is assigned ”tags” which are under the post title. If you need a quick answer to a gardening question give us a call or send a text to our customer support team – support[at]deep-roots-project.org AND 708-655-5299.
Deep Roots online store
See our online store for details about prices, ordering and delivery of raised beds, planter boxes, microbe-rich compost, worm castings, leaf mulch and more. We don’t sell traditional soil, since we use 100% compost as our growing medium. Our online store has 2 sections – (1) raised beds and planter boxes and (2) compost, worm castings, fertilizer.
Please contact our customer support team before placing an order online so we can assist you with the details and answer your gardening questions. You can pay by credit card in the store or by check.
(708) 655-5299 and support[at]deep-roots-project.org
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