Vegetable Seed Germination Temperatures and Phenology

Summer Lettuce Nursery Seedbed with Concept, De Morges Braun, New Red Fire and Loma lettuces.
Photo Bridget Aleshire. Lettuces are impossible to germinate if the soil is too hot. We use shadecloth (folded over on the right), a soil thermometer and ice cubes.

Vegetable Seed Germination Temperatures

Here is a table of vegetable seed germination temperatures. These apply to soil temperatures when you sow directly in the ground, and to air temperatures when you sow indoors in small containers. If your indoor air temperature is not warm enough when you want to sow your chosen crop (watermelons, anyone?) you can make a small warm place, or use a professional heat mat, or, for a small scale, germinate seeds in an instant pot! If you have one of these handy cooking devices, check the lowest setting. Perhaps labeled for making yogurt, it might be 91°F (33°C). Look at my chart below and see if your seeds will germinate at that temperature. You’ll have to experiment for the seeds which germinate well at 86°F (30°C) but not at 95°F (35°C). Don’t try this with spinach or lettuce! You might be surprised to see that some cool weather crops, like broccoli and cabbage, can germinate just fine at high temperatures!

You can see my chart is a work in progress, so if you can add any info, please leave a comment on this post. Bold type indicates the best temperature for that vegetable seed. The numbers indicate how many days it takes that seed to germinate at the temperature at the head of the column. Where I don’t know the number of days, I have put “Yes” if it does germinate at that temperature, “no” if I think it doesn’t and a question mark where I plain don’t know. I would love to know, so if you can resolve the uncertainties, please speak up! I’ve also used the words “best’, “min” and “max” which I hope are self-evident.

Vegetable seed germination

 

Ice cubes over newly sown lettuce seed, to help germination in hot weather. Shadecloth is folded open on the left. Photo Bell Oaks

Soil Thermometers

Soil thermometer.
Photo by Green Living/Taylor

To measure the temperature of the soil outdoors, I recommend a dial-type soil thermometer. Ignore the vague guidelines on Min/Optimal/Max and use the table above. The usual practice is to check the temperature at 9am each day, and if you are unsure, check again the next day. In some cases it is best to get 4 consecutive days of suitable temperatures, or even (in spring!) a few days of rising temperatures.

Harbinger weeds of spring and fall

Flowering Purple (or Red) Dead Nettle, with honeybee.
Credit Kathryn Simmons

The progress of emergence of different weeds in the spring shows us how quickly the soil is warming up. I wrote about that here. In that post you can see photos of flowering chickweed, purple dead nettle, and henbit.

Chickweed seedling.
Photo from UC IPM Weed Gallery

In the fall we will be waiting for the soil to cool enough to sow spinach. I have a blog post about this here, and also photos of those three weeds as seedlings, which is what we are looking for in the fall, as an indication that the soil has cooled down enough for them (and spinach!) to germinate.

Phenology

I have a post about phenology here.You can read some of the details of when to plant by natural signs. For instance, we sow sweet corn when white oak leaves are the size of a squirrel’s ear. I got excited this past weekend (April 10) when I saw wind-driven twigs on the ground with oak leaves definitely bigger than squirrels’ ears. But they were Red Oak, not White Oak.

The chart in that 2013 post has now got corrupted (at least it has on my screen), so here is a pdf

Phenology Record

 

A new bean bed with sunflower landmarks. When lilac is in full bloom, plant beans, squash, corn.
Photo Pam Dawling

Phenology records are a useful guide to when to plant certain crops, and a way to track how fast the season is progressing right where you are. Phenology involves recording when certain wild and cultivated flowers bloom, seedlings emerge, or various insects are first seen. These natural events can substitute for Growing Degree Day calculations. Certain natural phenomena are related to the accumulated warmth of the season (rather than, say, the day-length), and by paying attention to nature’s timetable you will be in accord with actual conditions, which vary from year to year, and are changing over a longer time-scale.

Keeping your own phenology record will help build resilience in the face of climate change. Ours might be interesting to you, but unless you live in central Virginia, you can’t use our dates. You do need to make your own. This can be a great home-schooling project, or a crew I-Spy competition, or a calming end-of-day walk around your gardens.

Hoophouse winter greens, transplanting spinach, crocus flowering

Russian kale, yukina Savoy and lettuce from our winter hoophouse .
Photo Wren Vile

Our hoophouse is bursting with winter greens. We just decided to hold back on harvesting our outdoor Vates kale and focus on the greens  which are starting to bolt in the hoophouse. That includes the last turnips (Hakurei, Red Round and White Egg), Senposai, tatsoi, Yukina Savoy, mizuna, Ruby Streaks, Scarlet Frill and Golden Frills mustards. Big but happily not yet bolting are the spinach, Rainbow chard and Russian kales. A row of snap peas has emerged. Time to stake and string-weave them!

The lettuce situation is changing as we are eating up more of the overwintered leaf lettuce in the hoophouse. The lettuces in the greenhouse have all gone, to make way for the flats of seedlings. Plus, we needed the compost they were growing in, to fill the flats. More about lettuce in February next week.

We have also cleared the overwintered spinach in one of our coldframes, so we can deal with the voles and get them to relocate before we put flats of vulnerable seedlings out there. The voles eat the spinach plants from below, starting with the roots. We had one terrible spring when they moved on to eat the baby seedlings when we put those out there. After trial and error a couple of years ago, we now clear all the spinach from one frame, then line the cold frame with landscape fabric (going up the walls a way too), wait two weeks, then put the seedlings out on top of the landscape fabric. The voles by then have decided nothing tasty is going to appear there, so they move on.

Spinach over-wintered in our cold frame
Photo Wren Vile

Outdoors, we have just started transplanting new spinach. We have four beds to plant, a total of  3600 plants, so we have to keep moving on that! We are trialing several varieties again, as we did in the fall. We have the last Tyee, alongside Reflect and Avon this spring. Inevitably things are not going perfectly according to plan. Yesterday I forgot to follow the plan, and we started with Avon and Tyee at opposite ends of a bed we had planned to grow Reflect in! Anyway, we are labeling everything and hoping to learn which have best bolt resistance. Watch this space.

We have grown our spinach transplants (as well as kale and collards) in the soil in our hoophouse, sowing them in late January. I wrote about bare root transplants in early January this year. You can find more links and info in that post. Growing bare root transplants saves a lot of work and a lot of greenhouse space.

For those relatively new to this blog but living in a similar climate zone, I want to point you to The Complete Twin Oaks Garden Task List Month-by-Month. It includes a link for each month’s task list. I notice from the site stats that some of you are finding your way there, but now there are so many years’ worth of posts it’s perhaps harder to find. Happy browsing!

Following on from last week’s mention of harbinger weeds of spring: chickweed, hen-bit and dead-nettle, I can now report that I’ve seen a flowering crocus (2/17), another marker on our phenology list. The average date for first crocuses here is February 8, so they are later than usual. I did notice however, that the foot traffic over the patch of grass has been heavier than usual.

Anne Morrow Donley sent me a link to WunderBlog®, the blog from Wunderground, my favorite weather forecast station, to an article by Bob Henson: This is February? 80°F in Denver, 99° in Oklahoma, 66° in Iceland, 116° in Australia. It includes a map of the Daily Spring Index Leaf Anomaly, Figure 1.

Image credit: USA National Phenology Network via @TheresaCrimmins.

Figure 1. An index of the seasonal progress of leafy plants shows conditions 20 days or more ahead of schedule over large parts of the South and Southwest as of Sunday, February 12. Image credit: USA National Phenology Network via @TheresaCrimmins.

The post has lots of other interesting weather info too. Thanks Anne!


I remembered another of the items lost in the hacked post a few weeks ago: My Mother Earth News Organic Gardening Blogpost on Heat Tolerant Eggplant Varieties made it into their 30 Most Viewed blogposts for 2016. I’ll be writing up more about those varieties, linking the 2016 results to the weather each week (especially the temperatures) and adding what I learn in 2017.