When to Pull the Flower Trigger

May 6th, 2025

   Now that we’re into May and usually past the point of killing frost, the big question on gardeners’ minds is exactly when is it “safe” to plant the summer flowers and vegetables?

Keep a cover handy in case a surprise frost comes along after planting the summer annuals.

   Those are the plants that will croak on frosty nights… things like zinnias, marigolds, and begonias in the flower garden and tomatoes, peppers, and cucumbers in the vegetable garden.

   You don’t want to plant those too soon and have your work and investment wiped out by a rogue cold night.

   On the other hand, most gardeners don’t want to give up time when their plants could be in the ground and on their way to peak beauty or a tasty harvest.

   Weather forecasts can help us decide at least a few days out, which is typically enough once we get to this point.

   What I do is wait until late April and then start looking at the 10-day forecast. If there’s nothing close to freezing overnight, I’ll go ahead and get an “early” start. If we’re looking at anything under 40 degrees, I’ll wait.

   That system usually works, but two things can still get you.

   One is that the weather forecast will be wrong.

   The other is that an unusually late frost will come along beyond the 10-day forecast.

   Knowing a few key climate-record stats also can help.

   The first one is our all-time latest spring-frost date. At least according to historical readings from Harrisburg International Airport, Harrisburg’s all-time latest frost date is May 11 (set in 1966). Temperatures that night went down to 31 degrees and haven’t gone lower after a May 11 ever since – until the first killing frost of fall, that is.

   Frost could happen here later than May 11, but it would take a record-setting event to do it.

   Harrisburg-area gardeners widely use this date (or the looser milestone of Mother’s Day) as their cue to plant summer annuals and summer vegetables.

   A second set of records involves the average date that our last spring frost occurs. That average has been creeping earlier and earlier over the past 25 years.

   If you go by what’s been happening since 2000, the average last killing-frost date in spring for the Middletown/Harrisburg area is April 11, according to National Weather Service records.

   First fall frosts also have been averaging later, meaning a usually longer growing time on the back end of the season, too.

   If you want to consider with recent spring norms, National Weather Service climate data from 2000 to 2024 places the median last killing-frost date for the Middletown/Harrisburg area at April 11.

   Here are the year-by-year last-frost 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), and 2024 (March 26).

   Note that last year, Harrisburg had a super-early last-frost date of March 26. But on the other hand, as recent as 2020, frost happened on the night of May 10. So those dates can vary widely from year to year – not to mention from area to area.

   Which brings me to another important caveat – just where you’re gardening.

   While a gardener living in concrete-and-asphalt protected Harrisburg might escape frost on a borderline night, a gardener living at a higher elevation in the open above Peters Mountain in Halifax might wake up to a blob of brown mush where the new red zinnias were the day before.

   That’s why gardeners in more northerly and outlying areas often wait not only until after Mother’s Day to plant but sometimes as late as Memorial Day.

   No matter where you live, though, it’s a good idea to keep some floating row cover or light-weight sheets or blankets handy. Those can be draped over tender plants already in the ground when a surprise freeze pops up, then removed the next day.

   Coverings like those only give a few degrees of protection, but often that’s all you need.

   To zero in on your exact area, the Dave’s Garden website has a free online tool that lets you type in your Zip code and see the record first and last frost dates for that area. It also gives date ranges for average frosts, the near-guaranteed frost-free ranges, and the average length of your specific frost-free growing season.

   Another caveat is that even short of a killing frost, some summer plants don’t do well in cool soil or during cool nights. They’ll just sit and sulk until the real heat arrives.

Vinca is an example of a flower that doesn’t like cold soil even if air temperatures don’t drop below freezing.

   Vinca is an example of an annual flower that often yellows when it’s grown in too-cool conditions. These sometimes never thrive after a chilly beginning.

   Zinnias, basil, okra, hot peppers, eggplants, and sweet potatoes are others that prefer solid warmth over borderline frost.

   The bottom line is that the tender-plant planting decision boils down to each gardener’s risk aversion as well as how antsy you are to get started.

   To get those young transplants off to a good start, make sure your greenhouse-bought annual flowers and vegetables are acclimated to the outside before going in the ground.

   Plants that are started and grown in greenhouses may never have seen the real light of day, especially if you’re buying them out of a covered retail greenhouse or similar indoor site.

   Going from this wind-coddled, light-regulated environment directly into the harsher conditions of an erratic Harrisburg-area spring can lead to “transplant shock.”

   The sudden change can cause wilting, bleached leaves, and even plant death when the shock is severe enough.

   Unless you’re sure the plants you’re buying are outdoor-acclimated, give them about five to seven days of increasing light and outdoor exposure. That lets the plants adapt before the added shock of exiting their pots and going into your lousy clay.

   Also helpful is planting on a cloudy day – or at least in the evening when the plants can “rest” in darkness instead of dealing with unfamiliar brilliant sunlight right off the bat.

   If you can also time it to plant right before a gentle, soaking rain, so much the better.

   Annuals appreciate going into loosened soil, ideally with a dose of compost mixed in ahead of time.

   It’s usually not necessary to fray out the roots of these little transplants before planting, as is recommended with larger perennials, shrubs, and trees. However, if you’ve bought aging transplants in which roots are heavily circling the pots or cell packs, try to unfurl them as best as you can before planting.

   Immediately after planting, water in your new plants to settle them and help soil fill in all around the root balls.

   A dilute amount of balanced fertilizer can be added to the initial watering.

See George’s list of top 10 favorite annual flowers

See George’s list of bottom 10 summer annuals

See George’s list of all-time favorite vegetable varieties