Tulips at the Capitol, April 14, at blue hour. (Rich Pezzillo via Facebook) Tulips at the Capitol, April 14, at blue hour. (Rich Pezzillo via Facebook)

Freeze warning late tonight into early Wednesday *

After this resilient winter, I can’t blame my fellow gardeners for wanting to get out there and get planting. But with a freeze warning in effect tonight, there are things to consider.

The risk of frost/freeze damage depends on what you have planted. If it is the value pack of annuals at the local store you can afford more risk than if it is those prize heirloom tomato seedlings. Just like playing the stock market, we all have different tolerances for how much risk we are willing to take.

In terms of what plants may or may not be at risk tonight, CWG reader Bryan Gimaldi posted some nice advice on Facebook:

Typical spring bulbs will be just fine. Tulips can usually take at least into the mid-upper 20s in full bloom, and daffodils and crocuses to the low-mid 20s. Other bulbs like Hyacinths probably similar to tulips. Since most of us probably won’t get much below 29-32 degrees, those should be fine.

Leaf buds on most trees, generally OK. Some maples (especially Japanese maples) can be fussy, but I think they’ll be alright unless you’re in a low lying spot. Up here in Columbia, they’re only at the bud swell stage anyway, and they’re more cold hardy at that point than they are after actual leaves have expanded.

Flowering trees…well…it depends. Most fruit trees in full bloom are OK right at 32. Things start getting hairy around 29-31 degrees, but duration of the cold matters, too. If we’re talking temps that remain above freezing most of the night, then dipping below around 4 or 5 am for a couple hours, they may well be fine. If it dips below freezing at midnight and stays there, even if the bottom never gets below 30, that could be a bigger issue. Flowering cherries, pears, and Redbuds have similar tolerance, but really you have to be talking about temps below about 27 or so before it’s a total loss on the blooms.

Veggies like tomatoes, beans, peppers-shouldn’t be planted yet except right in the District and maybe places like Alexandria, or near the water. Because they’re toast without protection.

Peas, lettuce, onions…probably fine.

If you already have sensitive plants in the ground, having a big roll of plastic and some plant stakes is a must. Spreading this over them near sunset will help to hold in the heat from the ground and allow them to survive a light one night frost. Just be sure to get that plastic off the next morning as soon as temperatures get back above freezing, or that protective cover will turn into an oven when the sun hits it. If plastic is not an option, a sprinkler can be used but that requires monitoring to avoid waterlogging the garden. A covering of mulch (2-4”) will not keep the vegetation from being burnt but is a very good idea with new plantings to help keep the cold from penetrating shallow roots.

Moving beyond tonight, Justin Grieser looked at the statistics pertaining to the date of final freezes a couple of years ago. His chart (below) shows, that if you are in the city, odds are highly in your favor to safely plant to your heart’s content. At Reagan National, the last spring freeze has varied from as early as Feb. 27 to as late as Apr. 21. However, for those of you  well to the north and west, the last half of this month is shaky and even early May is no safe bet. Dulles Airport has experience freezing temperatures in May 5 times in the last 30 years.


(Justin Grieser)

One of my favorite go to sites for looking at the odds of freezing temperatures is: http://davesgarden.com/guides/freeze-frost-dates/ , where you can enter your zip code for a truly localized sense of freeze risks.

Of course, it isn’t just about the freeze dates but also about the type of plants. I was pleased on my trip to the local garden store to see that they had mostly hardier annuals out for sale. These include some of my favorites for hardiness (even in the event of a light freeze, 28-32 F) like snapdragons, pinks, calibrachoa and, of course, the hardiest of the hardy annuals, pansies. Those are great to get in the ground now for much of the area. On the other hand, I was dismayed to see coleus already for sale which folds with barely a hint of frost. At least there were no impatiens out yet, which should be on planting hold until the frost odds are less than ten percent.

In any case, in another month this will all be a thing of the past.  Then we can start worrying about the eternal garden threats of dryness, heat, deer, weeds, pests, and on and on.