Tag: soil thermometer

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.

Colorado potato beetles, germinating lettuce, wheelhoes

A ladybug on a potato leaf, looking for pests to eat Photo Kathryn Simmons
Not a Colorado potato beetle: A ladybug on a potato leaf, looking for pests to eat
Photo Kathryn Simmons

Our spring-planted potatoes are just starting to flower, and I’m now in my weekly routine of monitoring for Colorado potato beetles. I walk the length of the patch and back, switching rows every 9 paces or so. I squish and count the adult CPB, and also count the larvae (I don’t always squish all of those). Did you know you can tell a squashed female CPB from a squashed male? The females are full of orange eggs, until you squash them, that is, then the orange eggs are all over your fingers!

Potato plants can tolerate 30-40% defoliation early in life, 10-60% defoliation during middle age, and up to 100% defoliation at the end of the season without reducing the yield.So don’t panic! But do pay attention. The Action Level for Colorado potato beetle is 1 adult per plant, and that for larvae is 2 per plant. Our rows are 265 ft this year, and the plants are 12″ apart, so I’m looking at 530 plants when I go down and back. So far, we’re doing great. Last week I found 12 adults. This week I found 5 adults and 41 larvae. I still don’t need a mass slaughter program. If we need to spray I’ll use Spinosad. For home gardens you can buy Monterey Garden Insect Spray from Seven Springs Farm in Virginia. Or elsewhere. It is OMRI approved but is very toxic to bees (and Eastern Oysters, should you need to know), so we only spray Spinosad at dusk when the bees have gone home, and make sure not to pour the rinse water from the sprayer into the drains (which go via our septic system into the creek). Generally I use the driveway as a large inert area to spread the wash water over.

But I only use Spinosad if we have to. Another part of my plan is timely mowing of the clover patch next to the potatoes. The clover mix was undersown in the fall broccoli last year, and in the spring we simply bush hog that patch every time the weeds are getting too successful compared to the clover. Or ideally, before that happens. We had flowering crimson clover there earlier, and mowed that. Now what we see is white clover. there is red in the mix too, but I haven’t seen that flowering yet. Our hope/plan is that when we mow the clovers, many of the beneficial insects move over to the potato patch and eat the CPB.

At the great link I gave earlier, Andrei Alyokhin provides good information on the life cycle of the CPB, lots of resources, and a lovely collection of Colorado potato beetle haiku (traditional Japanese poetry) written and illustrated by Mrs. O’Malley’s second-grade students from the Old Town Elementary School in 2008 or so, somewhere in Maine I think. The poetry gives new perspectives as we walk the rows searching for the wee beasties.

Germinating lettuce seed

Some weeks I wonder what to blog about. This week I was helped by a neighboring grower who asked me questions about how I germinate lettuce. Aha! A timely topic! We try to grow lettuce for harvest all year round, here in central Virginia, and exactly how we do that varies with the season.
New flats of lettuce seedlings Credit Kathryn Simmons
New flats of lettuce seedlings
Credit Kathryn Simmons

From January until mid-March I sow in flats in the greenhouse, with heating to get the seeds germinated, then good old solar energy to grow them to transplanting size.

From mid-March to the end of April I sow in flats in the greenhouse, without extra heat.
Spring lettuce bed. Photo Wren Vile
Spring lettuce bed.
Photo Wren Vile

From May to late September I use an outdoor nursery seedbed and do bare-root transplants (heat-tolerant varieties). The soil temperature does not vary as much as the air temperature, so I don’t worry about cool nights.

From June I put shade-cloth over the lettuce seedbed, and only sow in the evening.
In July and August I put ice on top of the newly sown rows, under the shade-cloth.
baby lettuce mix in the hoophouse. Photo Twin Oaks COmmunity
Baby lettuce mix in the hoophouse.
Photo Twin Oaks Community

After that I sow lettuce mix in the hoophouse. (Click the link to see my Hoophouse in Fall and Winter slideshow.)

New Seed-Starters Handbook

Knotts handbook
Knotts handbook

There’s a chart of germination at various temperatures in Nancy Bubel’s New Seed Starter’s Handbook and in the Knott’s Handbook for Vegetable Growers which is all online.The soil temperature range for germination of lettuce seeds is 35-85F, with 40-80F being the optimum range and 75F the ideal soil temperature. At 41F (5C) lettuce takes 15 days to germinate; at 50F (10C) it takes 7 days; at 59F (15C) it takes 4 days; at 68F (20C) only about 2.5 days; at 77F (25C) 2.2 days. Then time to germination increases: 2.6 days at 86F (30C); after that it’s too hot.

A soil thermometer soon pays for itself and saves lost crops and frustration. If it’s too hot, find a cooler place (put a seeded flat in a plastic bag in the fridge or on the concrete floor in the basement). Or cool down a small part of the world. That’s what I do when I sow in the evening, water with freshly drawn cold water, line up ice cubes along the seed rows, and cover with shade cloth.
And finally, to a tool we use a lot at this time of year (when the weather is dry enough for hoeing to be successful): our Valley Oak Wheel Hoes. We have two, both with pneumatic tires (rocky soil jars your wrists, think long-term). We have one with the standard 8″ blade and one with a 10″ blade. We use them for the aisles in our raised beds, for between rows of corn, anywhere without mulch. They are very energy efficient, compared to a hand hoe. And some of the crew treat he opportunity to wheel hoe as a chance for an aerobic workout. Others use a more moderate speed.Cover more ground with less effort, and hence, get more done before the weeds get too big to hoe.
The handlebar height is readily adjustable, the blade assemblies can easily be switched from one tool to another (buy one wheelhoe and several different width blades), there are other attachments such as  a small furrower, a one-sided hiller and a 3-tine cultivator. And there are plenty of replacement parts available: we just ordered spare blades after wearing ours down to narrow strips.9_zoom_1413981475