When is it safe to plant, and what's dead? (2024)

The winter that doesn't want to give up has our plant development running a week or two behind schedule this year. Spring will really get here for good one of these days ... and hopefully not jump right into summer.

So when is it safe to plant, how do you protect your plants from erratic temperature changes, and how to do you assess what's damaged in your yard?

When to plant the tender stuff?

This weekend marks the milestone of the Harrisburg area's average last killing frost of the spring.

"Average" means that by now we're done with frost in half of the years, while a frost still could occur in the other half. Given how erratic the weather has been lately, though, who knows?

We all could go ahead and plant those tender petunias and tomatoes this weekend if we knew this year was one of the halves where frost is done. But frost-free dates can vary by weeks from year to year.

Even in springs that warm up quickly and appear to be "safe" for planting by late April, a rogue freeze can appear out of nowhere as late as mid-May in the Harrisburg area and into late May in the outlying areas to the north.

What to do? If you're a gambler, you could go ahead and plant tender fare if 1.) you don't mind the threat of losing an early planting half the time, or 2.) you're willing and able to cover tender plants on frosty nights (which is possible only in mild dips below the freezing mark).

Most people elect to wait until our all-time latest killing spring frost, which is May 11 in the Harrisburg area. (It's late May in rural areas and in higher elevations of northern Dauphin County and beyond).

That's why you'll often hear gardeners using Mother's Day as their cue to plant summer annuals and warm-weather vegetables.

A compromise that I use is to wait until end of April and look at the 10-day temperature forecast. If there's nothing in the outlook even close to freezing, I'll take the plunge.

Keep in mind that many plants can tolerate freezing nights - even when newly planted - and can go in the ground well before Mother's Day.

Those hardier plants include trees, shrubs, evergreens, perennial flowers, cool-weather vegetables such as cabbage, onions and lettuce, and cool-weather annual flowers such as pansies, snapdragons and dianthus.

It's fine to plant those from late March through April.

Cold-wimpier plants that prefer to wait for frost threats to pass include warm-weather vegetables such as tomatoes, peppers and cucumbers, and summer annuals such as marigolds, zinnias, coleus and begonias.

Be ready with the protection

OK, so you plant some tender newcomers and then run up against a forecasted frost - rogue or otherwise.

Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.

The best way to protect them is to cover them overnight with a light-weight covering, such as a thin sheet or burlap propped up by stakes to create a little hut. That can trap enough soil heat to keep those tender leaves from freezing.

Even better is a product called floating row cover - available in most garden centers and through gardening catalogs - that's specifically made for draping over top of plants. These are very light and breathable, letting through both light and rain but giving plants a few degrees of extra warmth.

Floating row covers can be left on plants day and night until the weather warms. Heavier and denser covers like sheets should come off when temperatures rise above freezing in the morning.

If you've bought tender plants but didn't plant them yet, move them inside overnight. A garage or enclosed sun porch is ideal.

There's no need to cover plants that are adapted to spring cold when a late-April or early-May frost occurs.

These include spring-flowering bulbs (daffodils, hyacinths, tulips, etc.); perennial flowers already blooming or poking up (salvia, sedum, daylilies, etc.); cold-hardy vegetables (cabbage, broccoli, onions, spinach, lettuce, etc.), and shrubs that are producing leaves and/or blooming.

Dead plants? Maybe not...

In the meantime, gardeners are getting antsy about plants that look dead.

The most likely scenario is that your barren shrubs and yet-to-appear perennials are still dormant. They'll push out new buds and leaves when the lagging spring weather produces enough sustained warmth to trigger growth.

The second scenario is that winter's cold really did cause some damage that plants haven't yet had time to overcome.

This is most apparent on broadleaf evergreens such as nandina, boxwood, and cherry laurel that had leaves brown and maybe even drop due to cold winter winds. However, some plants really have suffered branch diebacks.

Dieback most often happens on plants that are borderline-hardy in our winters (osmanthus, crape myrtle, and camellia, for example) and on new plants that haven't had a chance to fully root or that weren't properly watered heading into winter.

In all of the above cases, the solution is patience.

For broadleaf evergreens and conifers that suffered browned or dropped leaves/needles, do nothing. Wait and give them a chance to grow new foliage. In a few weeks, they'll likely be as good as new.

Plants that suffered branch diebacks as opposed to complete death also will push out new foliage growth in the coming weeks.

The difference is that the new growth may come only from lower and inside the plants but not toward the top and branch tips. If that happens, just cut back the dead wood to where you're seeing new growth occur.

Some shrubs may have died back the whole way to the ground but will send up new shoots from around the base. This is because cold killed all of the top growth but not the roots.

Live roots mean the plant has a good shot at fully recovering, although you'll be starting all over when it comes to size and form.

The worst thing you can do now is get impatient and cut off live but dormant wood - or worse yet, dig up a plant for dead that would've made a comeback if given a few more weeks.

Species such as hardy hibiscus, crape myrtle, figs, and even butterfly bush are notoriously slow to get started in spring. It's not unusual for them to wait until May to show new life, lest they get hit with a rogue late frost.

A few years ago after a harsh winter and a slow-coming spring, some plants didn't begin growing until June. I even had a fig that I was sure was dead that finally sprouted new shoots at the beginning of July.

One way to tell if trees, shrubs, and evergreens are dormant or dead is to scratch off some of the bark and try bending a few branches. If the wood is brown underneath the bark, and if the branches/twigs are snapping instead of bending, the plant probably is a goner.

The lesson is don't be too quick to give up. Be patient and give them at least until Memorial Day.

For plants that truly are dead, think twice before replacing it with the exact same plant.

Try to get to the "root of the problem" (har-har) and nail down what caused the plant to die. If you put the same thing in the same spot under the same circ*mstances, you'll likely get the same result.

Among the many plant-killing issues to explore:

* Did the plant die because of rotting in wet clay?

* Was it a bug or disease?

* Did you unwittingly plant a wet-soil preferrer in the dry shade near a spruce tree?

* Was the site too windy or too open and the winter too cold for that borderline-hardy camellia?

* Did rodents strip the bark or chew the roots underground over winter?

Once you complete the autopsy, either plant a different plant that tolerates the situation you have (the easier approach) or change the conditions to increase the success odds of Try No. 2 for the same plant (usually harder and more expensive).

More when-to-do-what tips: George's "Pennsylvania Month-by-Month Gardening" book