The Perils of Early Planting & Cures for Hot Summers
Q. My father and I are having an argument. I have a Bachelor's in Botany, but he has more gardening experience than I have years in my life. Oklahoma City has been having an incredibly mild winter, and our last frost dates have been coming earlier and earlier (I think the most recent was around March 30th), so I think we should put our peppers and tomatoes out now* and cover them with blankets in case of a cold snap. Dad thinks we should wait until after the last frost date. I'm concerned that if we wait that long, summer will set in early as it often does here, and we won't be able to get any good harvests because of the heat. Is there a middle ground, or is one of us right?
- ---Lisa in Oklahoma City
A. Dad is right. In fact, he's doubly right because he's saying "after" the predicted date of your last Spring freeze. The nights are still often bitterly cold when a region reaches its 'last average frost date', and tropical plants like peppers and tomatoes don't need to actually freeze to suffer. Being exposed to nights in the mid-thirties can set their growth back by several weeks. And in 2014, the records show that it dropped to 34 in Oklahoma City on April 15th.
But Lisa does have a good point about summer heat. In a follow up email she explains, "we've had a lot of trouble with tomatoes in particular struggling or refusing to set fruit after daytime highs get up into the 90s, which is one of the reasons for me wanting to get them started so early."
*(Note: In her original email, when she said she wanted to put the plants out "now", it was still February—which means she had a bad case of Spring fever and cabin fever.)
Now, I understand the temptation when winter weather is warm, but you have to temper it with reality. It's going to drop close to or below freezing before she gets to April, and gardening is a game where cowards win. The "average last frost date" in my part of Pennsylvania is mid-May, but I often wait until June to plant. A good rule is to be ready to plant on your last frost date but check the ten day forecast and look at the nighttime temps. Anything below 45 is not ideal, and below 40 is really dicey.
And covering the plants with "blankets" would just crush them.
Now, it's perfectly acceptable to start your plants early, keep them indoors under lights or in a greenhouse until temperatures approach normal and then take them outside, but don't plant them immediately. Keep moving them up into bigger pots (filling in the extra space with compost and/or a chemical-free soil-free potting mix) and be ready to bring them back inside if you get a sudden cold spell. That way they'll be safely older, and much more acclimated to the outdoors when you finally do plant them.
And season extending devices are ideal for this situation. But it's easy to accidentally harm your plants with homemade structures, so I strongly suggest investing in a set of grow tunnels. These are row covers that are already attached to hoops so that they have a sturdy structure. They allow you to put tender plants outside weeks early—and you can use them to grow cool season crops like lettuce and spinach long into the winter—maybe even straight through to Spring.
Stored away carefully for the summer, they'd last many years in Oklahoma. Mine are several years old and have even survived some nasty winter ice storms.
Okay, now about hot summers…
In REALLY cranky areas like the deepest South, many gardeners treat summer like winter and plant early enough to take a break by mid-July or so. But there are lots of tricks you can play in 'normal hot' regions like Oklahoma. For instance, planting your tomatoes where they'll get morning sun, but afternoon shade. I always say that 'full sun' on a plant tag means full sun in the North, but it means 'please give me some afternoon shade' as you move South and Southwest. There are also varieties that are bred to better tolerate hot climes and to set fruit in higher temps; growers who swelter in summer should grow a few of these in their tomato mix.
And you can provide that shade artificially; every nursery and garden center does! Once things heat up, they run what's called shade cloth over their greenhouses to cut the intensity of the sun. Shade cloth is widely available and all you need to make it work is a big series of hoops to run it across. (Large rolls of floating row cover can also be adapted for this purpose.) Use quality materials that will last for years and the amortized cost would be very reasonable.But I have one big disclaimer for this specific region. Oklahoma is where the wind really does come "whipping down the plains". In a relentlessly windy area like this, you'll need to take the time to build a solid design and use extra strong materials—or it'll be 'gone with the wind' by August.