April 18, 2009

Watch those soil temperatures!

Last year I tried to start my vegetable garden from seeds in April...and most of the seeds never germinated.. Well, I did some research on seeds and soil temperatures and found out that the soil temperatures were too cold last year, due to unusually cold spring temperatures. As you remember, last spring was terrible...in some ways the worst (coldest) since 1918. Here are some interesting soil temperature facts.
Most seeds have minimum temperatures that allow germination. And if the temps are close to those minima, germination can be slow or delayed. Some seeds like it cold (peas and parsley can germinate at 40F), while others need warmer temps (cucumber, eggplant and peppers like it hot...60F). My disaster was with beans (which like 50F). And you don't want to know about my tomatoes. So how can you determine the soil temps...well you can stick a thermometer in there (try about 1 inch in)...or there are web sites ...such as the Washington AgWeather network, controlled by WSU (see example above and the web link).
Right now the soil temp of the upper inch or so is approaching 50F in much of western Washington. Want to protect your seedlings and seeds from cold...add mulch. A few inches of mulch can greatly lessen the daily (diurnal temperature range). So it will take out the lows, but also lessen the highs. Cold air tends sink into hollows...so picking a higher location in your yard or even a raised bed can help. And soil temperatures are substantial higher with southern exposures...and particular on the south sides of buildings. Covering your plants with clear plastic..making a small greenhouse... really helps since the plastic lets the sun in, reduces the loss of infrared radiation to space, and lessens the amount of mixing out of warm surface air during the day.
Anyway, time for me to hit the nursery. But a warning...it will be warm on Sunday and Monday, but a cooling trend will develop Tuesday through Thursday. Too cool for tomatoes...which are notorious for being damage by cold. I have found some of the cold-favoring varieties really help.... cliff

PS: I will be at Costco in Kirkland from 11-1 PM signing their books


  1. You're right, last spring was terrible - too much sun! :)

  2. Hey Cliff, what Garden nursey or nurseys will you be going to? Feel free to come on up to Molbaks here in Woodinville.

  3. True, but no one with any sanity tries to germinate eggplants, peppers or tomatoes outside around here. By the time it's warm enough to germinate them, they don't have enough time to mature. Germinate inside (or buy starts), transplant outside.

    Of the top of my head, beans, corn and the cucurbits are the only summer crops people try to germinate outside here. Cucurbits you can easily transplant, beans can be hard to go the transplant route, depending on how many you want, and corn practically demands you seed outside because you need a great many plants spaced just so to get full ears.

    My spinach, by the way, is very happy.

  4. I keep forgetting about your blog. I just bookmarked it. Thanks.

    I for one start almost all my seeds (except root crops) inside after last year. 3 inches of snow April 21, that's insane! Thankfully I put up my hoop covers and everything thrived last year. This year the hoop covers are still up from fall for that very reason!

    Thanks for the heads up. The tomato plants (my wife won't let me call them seedlings anymore) may not go out til next weekend now despite the nice weather we're having.

  5. I'm putting down cash; I bet it doesn't get to 70 over the next few days. And by "here" I mean Seattle proper - it's sure to be warmer in the foothills. The high layer of clouds today looked like the quintessential Seattle "summer". You know the one, where we get excited that some sun has been spotted at the intersection of Dexter & Denny but the real summer weather doesn't start until July 5th. You know, right after the big fireworks and bbq.

    I didn't see any snow left on Tiger on Thurs. - anyone else spot some?

  6. In the past, I've had luck starting tomatoes early (before Mother's Day) using something called Wall O' Water. It's not produced anymore, but there's another similar product available:


    You can also emulate the Wall O' Water effect by arranging a bunch of clear plastic water or soda bottles around your plant start.

    The idea is to keep the seeds warmer at night using the heat stored in the water's thermal mass. And during the daytime, the water works like a greenhouse window.

  7. Here in the PNW the best bet (and cheapest bet) is to start seeds indoors and put out plants once it warms up. As Cliff said peas are happy in cool soil and we plant ours on the traditional St. Patrick's day. The Seattle Tilth sale is coming up in May and they are a great source of veggies, get there early:
    Tilth Edible Plant Sale
    When: Saturday, May 2-Sunday May 3, 9am-3pm each day

  8. show me the money

  9. Grrr - Saw that blue sky this a.m. and knew I was busted!

    I was framed. I swear.


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