Sowing Dates & Soil Temperature

See links below from Harvest to Table.com with plant-specific advice including soil temps.

Min/Opt/Max temps from IFAS

MINIMUM Soil Temperature for Germination

• 32° – Lettuce, Onion, and Spinach
• 40° – Beet, Broccoli, Cabbage, Carrot, Cauliflower, Parsley, Pea, Radish, Swiss Chard
• 50° – Sweet Corn, Tomato
• 60° – Lima bean, Snap Bean, Cucumber, Okra, and Pepper
• 65° – Squash

OPTIMUM Soil Temperature for Germination

• 70° – Celery, Parsnip, and Spinach
• 75° – Asparagus, Endive, Lettuce, and Pea
• 80° – Lima Bean, Carrot, Cauliflower, Onion, Radish, Tomato, Turnip
• 85° – Snap Bean, Beet, Broccoli, Cabbage, Parsley, Pepper, Sweet Corn, and Swiss Chard
• 95° – Cucumber, Okra, Squash

MAXIMUM Soil Temperature for Germination

• 75° – Lettuce and Spinach
• 85° – Lima Beans, and Pea
• 95° – Snap Bean, Beet, Broccoli, Cabbage, Carrot, Cauliflower, Eggplant, Onion, Parsley, Pepper, Radish, Swiss Chard, and Tomato
• 106° – Cucumber, Okra, Squash, Sweet Corn

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For bok choy planted in the fall or the spring, the optimal soil temperature for direct sown germination is 40-75 F. (4-24 C.). Space the seeds 6-12 inches apart. Keep the bed moist. Bok choy is ready to harvest in 45-60 days. Read more at Gardening Know How: Bok Choy Planting Time: When Do I Plant Bok Choy 

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From Harvest to Table.com

• VEGETABLE SEED GERMINATION SPECIAL REQUIREMENTS
• SEED STARTING SPECIFIC CROPS
• VEGETABLE SEED GERMINATION TEMPERATURES – Extracted table is below.

Crop	Germination Minimum Soil Temperature (°F)	Germination Optimum Soil Temperature Range (°F)	Germination Maximum Soil Temperature (°F)	Days to Germination at Optimal Temps
Asparagus	50	60-85	95	14-18
Bean, lima	60	75-85	85	4-10
Bean, snap	60	75-85	95	4-10
Beet	40	60-85	95	4-10
Broccoli	40	60-85	95	7-10
Brussels sprouts	40	60-85	95	3-10
Cabbage	40	60-85	95	5-10
Carrot	40	65-85	95	6
Cauliflower	40	65-85	95	4-10
Celery	40	60-70	95	10
Chinese cabbage	40	60-85	95	4-10
Collards	40	60-85	95	5-10
Corn	50	65-95	105	4-10
Cucumber	60	65-95	105	5-7
Eggplant	60	75-85	95	10-15
Endive/Escarole	35	60-75	85	10-14
Garlic	35	65-85	95	7-14
Kale	40	60-85	95	5-7
Kohlrabi	40	60-95	105	5-10
Leeks	35	65-85	95	8-16
Lettuce	35	60-75	85	2-10
Muskmelon	60	75-95	105	4-10
Mustard	35	60-75	85	4-6
Okra	60	85-95	105	7-12
Onion	35	65-85	95	4-12
Parsley	40	65-85	95	5-6 weeks
Parsnip	35	65-75	85	5-28
Pea	40	65-75	85	5-7
Pepper	60	65-75	95	7-10
Pumpkin	60	85-95	105	4-10
Radicchio	35	60-75	85	5-7
Radish	40	65-85	95	4-10
Spinach	35	65-75	75	6-14
Squash	60	85-95	105	7-10
Swiss chard	40	65-85	95	7
Tomato	50	65-85	95	5-7
Turnip	40	60-95	105	3-10
Watermelon	60	70-95	105	4-10
Zucchini from here	70	85		7-10

Below is the above table sorted by Maximum Germination Temperatures for Fall Garden Planning

Crop	Germination	Germination	Germination Max Soil Temp.
	Minimum Soil	Optimum Soil
	Temperature	Temperature
		Range (°F)
Corn	50	65-95	105
Cucumber	60	65-95	105
Kohlrabi	40	60-95	105
Muskmelon	60	75-95	105
Okra	60	85-95	105
Pumpkin	60	85-95	105
Squash	60	85-95	105
Turnip	40	60-95	105
Watermelon	60	70-95	105
Asparagus	50	60-85	95
Bean, snap	60	75-85	95
Beet	40	60-85	95
Broccoli	40	60-85	95
Brussels sprouts	40	60-85	95
Cabbage	40	60-85	95
Carrot	40	65-85	95
Cauliflower	40	65-85	95
Celery	40	60-70	95
Chinese cabbage	40	60-85	95
Collards	40	60-85	95
Eggplant	60	75-85	95
Garlic	35	65-85	95
Kale	40	60-85	95
Leeks	35	65-85	95
Onion	35	65-85	95
Parsley	40	65-85	95
Pepper	60	65-75	95
Radish	40	65-85	95
Swiss chard	40	65-85	95
Tomato	50	65-85	95
Bean, lima	60	75-85	85
Endive/Escarole	35	60-75	85
Lettuce	35	60-75	85
Mustard	35	60-75	85
Parsnip	35	65-75	85
Pea	40	65-75	85
Radicchio	35	60-75	85
Spinach	35	65-75	75
Zucchini from here	70	85

Fall Germination Vs Soil Temps

The dates in the table below were produced by a spreadsheet calculator from Johnny’s Seeds website. The calculator determines planting dates for crops to be planted outside, without additional protection, for fall harvest.  All are crops that are frost-tolerant and taste great in cooler weather.

The calculator uses this formula:
Days to Maturity (they used varieties recommended for fall/winter)
+ Average Harvest Period
+ Fall Factor (takes into account the slowing of growth that results from cool weather and short days in fall, amounting to about 2 weeks)
= Days to Count Back from First Frost Date

The Optimum and Maximum Temperatures came from the page at this site that cites the data sources.

Based on the average first frost date of 11/30/2019
Crop	Plant NLT **	Opt Soil Temp	Max Soil Temp
Brussels Sprouts (TP)***	8/2/2019
Cabbage, storage (TP)	8/7/2019	85	95
Leeks (TP)	8/7/2019	75	95
Cabbage, fresh eating (TP)	9/1/2019	85	95
Carrots (DS)	9/5/2019	80	95
Broccoli (TP)	9/7/2019	85	95
Cauliflower (TP)	9/8/2019	80	95
Onions (DS)	9/11/2019	80	95
Beets (DS)	9/18/2019	85	95
Greens, Asian, full size (DS)	9/23/2019
Lettuce, heads (DS)	9/23/2019	75
Cabbage, Chinese (TP)	9/24/2019	85	95
Radish, Daikon (DS)	9/26/2019		85
Swiss chard, bunching (DS)	9/27/2019	85	95
Turnip, purple top (DS)	9/27/2019
Lettuce, baby (DS)	10/3/2019	75	85
Spinach (DS)	10/7/2019	70	75
Turnips, salad (DS)	10/8/2019
Radish, round (DS)	10/25/2019	80	95

** The date calculated is the date to plant outdoors; crops that should be direct-sown are marked DS. Those marked TP should be set out as transplants on the date shown, so seeds should be started 4 weeks before the planting date.
*** Brussels sprouts should mature after frost, so no additional time is added for the harvest period or fall factor.

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From Pam Dawling. Updated and posted in her blog 4/21/2021. These apply to soil temperatures when you sow directly in the ground, and to air temperatures when you sow indoors in small containers.

Pams-Vegetable-seed-germination