SPOKANE, Wash. — We're well into Spring and have seen a slew of 70 degree days already in Spokane. The weather's been very nice, albeit dry, and by May I think we're all chomping at the bit to get our gardens planted by now.
But is it still too early? The short answer is no. By Mother's Day, the second week in May, it's fairly safe to go ahead and start planting without a high risk for an incoming frost.
For the following reasoning, I should make it clear that I am a meteorologist, not an expert gardener. So my thought process is a bit more analytical. While, yes, we've seen a bunch of days above 70 degrees already, it's not the high temperatures we need to monitor, it's the lows.
Nights in April are just too cold too frequently to realistically be able to get a head start on planting. That average low temperature rises from 34 degrees to 40 throughout April. A month were we can see drastic changes in weather not just week-to-week, but day-to-day.
The average final day of a frost in Spokane (32 degrees or lower) is April 29! (Over the past 30 years.) That means that half the time, the last frost is before that date, half the time it's after. A frost in early May isn't all that uncommon. The last time it happened was each year from 2010 to 2013. In fact, in 2010 we had a frost on May 24, the fourth latest frost on record.
But by the second week in May, the likelihood of a frost drops, making it safer to plant. But you just have to watch for any cold snaps through May, where a blanket at night for the freshly planted flowers will protect them.
One saying I've heard around town is that it's "safe to plant once there is no snow on Mt. Spokane. At a peak of 5,886 feet, the reference point is easy to spot and the crest of the mountain often holds some snow depth well into the Spring.
But I find that waiting until there's no snow on Mt. Spokane will often be "too safe". Take this year for example, the snow has been melting steadily without any significant Spring snowfall to keep the depth any higher than normal, and there's still a decent amount at the highest elevations.
What I'm trying to say is that if you're waiting until the snow is completely gone, you might be waiting until mid-June some years.
Year-to-year, Spokane's last freeze was on April 12 (for now). This would be one of the earlier final freezes of the year.
In summary, April is too early and too risky to start planting, even if the afternoons start getting warm. The second week of May, or around Mother's Day is a good starting point. Listen and watch for the low temperatures forecasted just in case we get a late cold snap, that way you can protect any vegetables or flowers from perishing so early in the season.