With last week's string of unseasonably warm temperatures, many green thumbs are itching to get a jump start on their spring gardens. But is it safe to plant outdoors yet?


What You Need To Know

  • The average last spring frost typically occurs in late April or early May

  • Rhinelander and parts of northern Wisconsin have seen frost later than Memorial Day

  • A cold and wet April led to delays in planting crops across the state

It seems like we jumped right from our cold, wet April straight into summer last week with temperatures soaring into the 80s and 90s across Wisconsin. Although temperatures have dipped a bit this week, it's prime time for planting.

Average last frost/freeze

Most cities around Wisconsin experience their last at or below-freezing temperature in late April or early May. 

Milwaukee typically experiences the last spring frost around April 23, while cities much farther north, like Rhinelander, can see frost up to or even later than Memorial Day.

For most of the state, we are finally past our average last spring frost date for the year.

Heading into the end of May, the potential for a frost or freeze is low. Temperatures will stay near or just slightly below average for at least the next week, with highs in the 60s and 70s and overnight lows in the 40s and 50s.

Long-term weather models also suggest that will be the case right through the end of the month.

Even though Rhinelander's average last frost date is May 25, the potential for a frost or hard freeze is very unlikely.

Wisconsin crop progress

If you feel like you may be behind when it comes to planting, you are not alone. Thanks to the cold and wet pattern we dealt with through most of April and early May, Wisconsin farmers are running about two weeks behind on spring tillage.

While potato planting is about 69% complete, corn planting is at just 34% and soybean planting is only 26% complete. 

Thankfully, the weather looks to cooperate for Wisconsin farmers this week with just a couple of chances of rain in the days ahead.

What is safe to plant now?

Since we are well past our average last frost/freeze in most places, it is finally safe to plant most annuals and perennials in your garden.

However, perennials like milkweed, phlox or hibiscus should not be planted until one to two weeks after the last frost date in order to ensure that they develop a strong root system.

Most annuals are fairly hearty and can be planted anytime after the last frost date, but some varieties such as impatiens, sunflowers and zinnias thrive best when planted one to two weeks later.

According to the Master Gardener Program at the University of Wisconsin-Extension, early May is the best time to transfer broccoli, head lettuce, cauliflower, onions and parsley plants into the ground.

You will have more success waiting until late May to transfer your celery, melons, pumpkins, winter squash, tomatoes, eggplant and peppers.