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Min/Max thermometer. It’s worth having one of these (and not a digital one either – they don’t last very long). Blue liquid gets pushed up by the mercury (which is in a U shape). Left shows nightly low. Right shows daily high. Reset using a magnet. Last night? A chilly 25.

Spring is in the air, and everywhere you see helpful garden planting memes (graphic pictures and text with quick easy to digest visuals) that really aren’t all that helpful, and sometimes are flat out wrong. So I’ve recently made my own “when to plant your veggies” meme (see bottom of post).

But first, a bit of background for those really new to gardening. In general, garden vegetables/herbs can be divided up in to two categories. Those that can take a frost, and those that can’t.

Frost tolerant plants include anything in the brassica, aka mustard, aka cabbage, aka cruciferous family (which includes cabbage, cauliflower, broccoli, kohlrabi, brussel sprout, kale, collards, mustard greens, radish, arugula, turnip, rutabaga, bok choy, and probably a few others I missed); all the different types of lettuce; the onion family (including shallots, leeks and garlic); the parsley family (parsley, dill, carrots, celery, celeriac, fennel, and parsnips – though they are happier with a bit more heat), peas and garbanzo beans (but definitely NOT the green and shell beans), and the spinach/chard/beet group. It helps if you start thinking of your garden by plant family. That way when you want to try something new, but aren’t quite sure how to treat it, you’ll likely be in the ballpark if you know the family its in.

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Greenhouse is currently rocking the cool season transplant crops, including a lot of kale, kohlrabi, cabbage, cauliflower and broccoli, plus lettuce and many kinds of onions.

Warm season “I will die and make you very very sad if I am exposed to a frost” plants include the nightshades (tomato, tomatillo, peppers, eggplant, ground cherry, and potatoes – though the part underground will often survive to resprout, and so are often planted before the last frost date), the summer and winter squash (including pumpkin), all green and shell beans, the melons (including watermelon), cucumbers (which are actually in the melon family!), sweet potato (related to morning glory!) okra (in the mallow family – who knew?),  basil (actually a type of mint!) and sweet corn.

When is your last frost date? Well, you can google “last frost date zip code” and find different databases out there to give you an answer. Your local extension office is often a good source as well (though not ours – sad face). That said, given our ongoing climate shift (which really translates to more volatility), and the fact that these databases are often using the last 50 years of data rather than the last 10, you might find the date (which is really just a statistical guess) to be more conservative than you think is realistic. For Walla Walla, dates from different sources range from April 19th to May 10th. I tend to write down frosts in April/May so I can track the last one of the spring. Here’s what I have for the past 5 years:

2012: May 11 (31 degrees)
2013: April 22, 23, 24 (28 to 32 degrees)
2014: March 22 (24 degrees)
2015: April 15th (32 degrees)
2016: March 18th (24 degrees)
2017: April 3rd (25 degrees). Last one? We’ll see.

We tend to be, on our farm, about 5 degrees cooler than the airport weather station’s recorded temperature. It helps that I have a min/max thermometer so I’ve noticed that trend over the years.

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Lacinato (aka Dino) kale. Perfectly happy with the cooler temperatures.

So, a LOT of variation, and only in one year out of 5 has that May 10th cut off date been a good one. Haven’t been writing this stuff down? I LOVE Weather Spark for looking up historical weather data for the year for a specific area. I print out each year and file it for comparison. You can look at different years by selecting them in the right hand column.

But there are other things to take into consideration. Yes, onions and beets and radishes can be planted really early. But if they go through too many frosty nights, they start thinking they have been through a winter and almost immediately go to seed when the weather warms up. I once had a whole BUNCH of beets never make a bulb and go directly to seed, because I planted them way too early. Lesson learned. Onions will do something similar, especially if you plant the “onion sets” from the store. Those onions think they have already gone through one growing season. It’s very hard to keep them from sending up a seed head if you plant them too early. (Actually, I found that it was hard to get them to not send up a seed head no matter what I did, so I started growing my onions directly from seed instead).

Every seed has an “optimum” germination soil temperature, where you will get the max number of seeds germinating in the shortest amount of time.

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Graphic clipped from tomclothier.hort.net/page11.html, something I ran across and saved years ago.

That temperature is always slightly warmer than the temperature in which growth happens. So if you can get a seed to germinate, you can often keep it growing (though all plants have a lower growth threshold as well, which is why you often plant out a vegetable start from the big box store too early and have them just sit and do almost nothing for weeks and weeks!) This is part of why I start some cold season plants in the greenhouse. Once I get them to germinate, I’m pretty confident they will grow once in the ground, even if it’s still a bit chilly.

Onions are an interesting example. They have a 90% germination rate at 32 degrees. But it takes them 136 days to do it! At 68 degrees, they have a 99% germination rate in 5 days. Sometimes, its worth compromising on the optimum germination rate in order to get a jump on the season. Optimum for watermelon is 95 degrees, when they have a 96% germination rate in 3 days. But for most of us, our soil NEVER gets that warm, and if it did, you’re probably so far into the season that your melons won’t mature before you got a frost anyway. At 68 degrees, you have a 94% germination rate in 12 days. A worthy trade off.

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Baby tomatoes. MUST be protected from frost, and won’t grow if your greenhouse temperatures are too low. But oh they are SO worth all the coddling early in the season when you harvest that first tomato in mid to late July. I start mine 8 weeks before planting out, on March 15th.

This is also, of course, why we tend to start tomatoes, peppers and their kin in greenhouses 8 weeks or so ahead of when we can set them outside in the ground. We NEED that extra 8 weeks of growing time if we want to achieve optimum harvest during our growing season.

A note on soil temperatures. When you look at seed temperature germination charts, they are based on soil temperature, not air temperature. But most home gardeners don’t run around with a thermometer stuck in the ground every day measuring and recording soil temperature (though I’ve been known to do that 🙂 ). You can get a rough estimate of your current soil temperature from looking at your weather report on air temperature for the last three days. Take the high and low for the day, add them together, and divide by 2 to get the daily average. Do that for the last three days. Then take the average of your three averages to get an idea of your soil temp in that first inch or so of soil.

So, if my last three days were a high/low of:

  • 63:41 (average 52)
  • 56:43 (average 49.5)
  • 53:36 (average 44.5)

Then my three day average is 48.66. So my soil temperature is around 49 degrees right now. Pretty cool huh!

So, here at last, after all this babbling, is my “when to plant your garden” chart. This is based on various internet resources, but also my own experience. Let me know if you have any input. I’d be happy to update it. Every year I learn something new!

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Miles Away Farm Blog © 2017, where after our 25 degree night last night, we’re happy we moved the peppers and tomatoes out of the greenhouse and into the garage for that 8 hour period! 

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