Navigating Our New Gardening Weather

April 23rd, 2024

   When is it “safe” to plant the tomatoes and summer flowers?

Our frost dates are changing. It may be possible to get an earlier start to the season… but keep the row covers handy!

   That’s a question gardeners wring their compost-stained hands over every year at this time – especially the eager ones who don’t want to miss a day of potential frost-free grow time.

   Not that long ago, Harrisburg-area gardeners could rely on the long-held rule of thumb that the last frost of spring usually happened somewhere around early May. And so the word on the street was that you waited until after Mother’s Day to pull the trigger on the marigolds, petunias, tomatoes, and such.

   However, if you go by what’s been happening since 2000, that Mother’s Day milestone is outdated and in most years gives up two weeks or more of frost-free time.

   According to National Weather Service climate data from 2000 to 2023, the average last killing-frost date for the Middletown/Harrisburg area is April 11.

   Last spring, the Middletown/Harrisburg area had its last killing frost on April 9.

   You might be surprised at how early those dates are, but keep in mind that those are average and one-year figures.

   What can happen in any given spring is far iffier.

   That’s why gardeners – at least the cautious ones – still look to the all-time latest frost date when timing their planting of summer vegetables and annual flowers.

   In the Harrisburg area, that date is May 11 – or in other words, the Mother’s Day guideline.

   In the outlying regions to the north of Harrisburg, it’s still possible to get a rogue late frost into late May.

   The bottom line is that it’s a matter of how much risk you want to take before pulling the tender-plant trigger – or if you have enough sheets or floating row cover to cover those plants if you guess wrong.

   If you’re a data-driven person, the National Gardening Association has an interactive online tool that lets you type in your Zip code and then see what the odds are that the temperatures will go down to 32 degrees or below at various points in March, April, and May.

   And if it’s recent track record that you like better, the National Weather Service has a chart showing the actual last dates that temperatures went down to at least 32 degrees in the Middletown/Harrisburg area over the past 23 years.

   Here are those dates:

2000 (April 13)

2001 (April 29)

2002 (April 7)

2003 (April 8)

2004 (April 16)

2005 (March 22)

2006 (April 10)

2007 (April 11)

2008 (April 3)

2009 (April 13)

2010 (March 27)

2011 (April 2)

2012 (March 30)

2013 (April 21)

2014 (April 16)

2015 (April 25)

2016 (April 10)

2017 (March 23)

2018 (April 21)

2019 (April 3)

2020 (May 10)

2021 (April 23)

2022 (April 11)

2023 (April 9)

   One strategy I’ve used for years is to wait until around the last week of April and then look at the 10-day forecast. If there’s nothing even close to the 32-degree overnight mark, then I plant. If the 10-day forecast is showing even a brush with frost, I wait.

Here’s what happens when you plant a tender annual too soon and frost shows up.

   At the back end of the growing season, our first killing frost of fall has been getting later.

   Last fall, our first 32-degree night occurred on Nov. 2, which happens to be the exact average for the period between 2000 and 2023.

   If you count up the days between our recent average last frost of spring (April 11) until our recent average first frost of fall (Nov. 2), we’re up to an average season of 205 frost-free growing days.

   The fall before (2022), the first killing frost didn’t happen until Nov. 14.

   This past winter, by the way, was another way-warmer-than-usual one. And once again, it coerced many of our landscape plants into a premature start before some “normal” temperatures during the second half of March slowed things down.

   On the whole, the winter of 2023-24 wreaked very little cold havoc on plants. Landscapes are filling in nicely now.

   We had nothing even close to zero degrees this winter. The lowest temperature recorded at Harrisburg International Airport’s weather station this winter was 14 degrees, which occurred Jan. 8 and again on Jan. 17.

   February’s lowest reading was 20 degrees, and March got no colder than 25 degrees.

   On the other hand, we had 59-degree days in both December and January, a high of 62 in February, and a balmy 76-degree day on March 14.

   Putting it all together, we had four straight normally cold-weather months that each averaged at least 4.2 degrees warmer than average (December was 5.3 degrees warmer than normal, January was 4.2 degrees warmer, February was 4.9 degrees warmer, and March was 4.5 degrees warmer).

   This kind of sustained recent warmth is pushing our climate into something more akin to what used to be Baltimore’s growing conditions.

   The new Plant Hardiness Zone Map that the U.S. Department of Agriculture released at the end of 2023 confirmed that our winters aren’t as plant-killing cold as they used to be.

Read George’s post on what the new Hardiness Zone Map means to gardeners

   The upshot is that we have better odds these days of getting away with borderline-hardy trees, shrubs, evergreens, and perennials that likely would have died in Grandma’s day.

   But just as with the frost-date issue, an important thing to keep in mind as you head out to garden centers is that the USDA Zone Map measures the average lowest temperatures in winter over a set period of time.

   It doesn’t mean our days of sub-zero winter temperatures are gone for good.

   It’s still very much possible for a rogue “Arctic clipper” or “polar vortex” to sweep in and knock a night or three down to well below our “average” low.

   And as any astute gardener knows, all it takes is one night of super-cold to kill a plant whose genes aren’t adapted to that kind of winter malice.

   On the other hand, the changes are turning once-iffy plants into surer bets and opening the door to new plants that we could not have grown here before – at least not without extraordinary winter-protection efforts.

Aucubas are becoming a surer bet in our winters.

   Nandinas, crape myrtles, figs, cherry laurels, and aucuba are among the species fast becoming “normal” fare for south-central Pennsylvania gardens.

   Moving from “no way” to “decent chance” are such previously “southern” plants as osmanthus (tea olive), Carolina jessamine, photinia, reblooming azaleas, “skip” and Portugal laurels, Chinese fringe flowers, agapanthus, alstroemeria, and, yes, even some of the cold-hardier types of camellias, gardenias, and palms.

   If you’re an envelope-pusher, you’ll be more comfortable in shopping for plants like those. Local garden centers might even be more willing to bring in warmer-climate plants that they didn’t previously carry.

   If you’re less of a risk-taker, though, you might still want to stick with plants listed as winter-hardy to Zone 6b and lower on the USDA Hardiness Zone scale.

Read George’s PennLive column on how to garden in the “new normal” climate

Read George’s PennLive column on how our warming climate is affecting plants