Sowing Dates & Soil Temperature

Jump here to info for annual flower seed germination.


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

For both bok choy planted in the fall or the spring, the optimal soil temperature for direct sown germination is 40-75 F. (4-24 C.). The soil should be well draining and rich in organic material. Space the seeds 6-12 inches (15-30.5 cm.) 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 


From Harvest to Table.com

CropGermination
Minimum Soil
Temperature (°F)
Germination
Optimum Soil
Temperature
Range (°F)
Germination
Maximum Soil
Temperature (°F)
Days to
Germination
at Optimal
Temps
Asparagus5060-859514-18
Bean, lima6075-85854-10
Bean, snap6075-85954-10
Beet4060-85954-10
Broccoli4060-85957-10
Brussels sprouts4060-85953-10
Cabbage4060-85955-10
Carrot4065-85956
Cauliflower4065-85954-10
Celery4060-709510
Chinese cabbage4060-85954-10
Collards4060-85955-10
Corn5065-951054-10
Cucumber6065-951055-7
Eggplant6075-859510-15
Endive/Escarole3560-758510-14
Garlic3565-85957-14
Kale4060-85955-7
Kohlrabi4060-951055-10
Leeks3565-85958-16
Lettuce3560-75852-10
Muskmelon6075-951054-10
Mustard3560-75854-6
Okra6085-951057-12
Onion3565-85954-12
Parsley4065-85955-6 weeks
Parsnip3565-75855-28
Pea4065-75855-7
Pepper6065-75957-10
Pumpkin6085-951054-10
Radicchio3560-75855-7
Radish4065-85954-10
Spinach3565-75756-14
Squash6085-951057-10
Swiss chard4065-85957
Tomato5065-85955-7
Turnip4060-951053-10
Watermelon6070-951054-10
Zucchini from here70857-10

Fall Germination Vs Soil Temps

The dates in the table below were produced by a spreadsheet calculator from Johnny’s Seeds web site. 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   
CropPlant NLT **Opt Soil TempMax Soil Temp
Brussels Sprouts (TP)***8/2/2019  
Cabbage, storage (TP)8/7/20198595
Leeks (TP)8/7/20197595
Cabbage, fresh eating (TP)9/1/20198595
Carrots (DS)9/5/20198095
Broccoli (TP)9/7/20198595
Cauliflower (TP)9/8/20198095
Onions (DS)9/11/20198095
Beets (DS)9/18/20198595
Greens, Asian, full size (DS)9/23/2019  
Lettuce, heads (DS)9/23/201975 
Cabbage, Chinese (TP)9/24/20198595
Radish, Daikon (DS)9/26/2019 85
Swiss chard, bunching (DS)9/27/20198595
Turnip, purple top (DS)9/27/2019  
Lettuce, baby (DS)10/3/20197585
Spinach (DS)10/7/20197075
Turnips, salad (DS)10/8/2019  
Radish, round (DS)10/25/20198095

** 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.


Flower Seed Germination

For which flowers can be sown in the garden go to Iowa State University Page Direct-Seeding Annual Garden Flowers.

The table and paragraphs below were extracted from Annual Flower Seed Germination Guide by Richard Jauron, Iowa State University, Department of Horticulture. But, it does not include Marigold Seed: Marigold seeds generally do not germinate until soil temperatures reach 65 degrees Fahrenheit, though to be safe you should wait until they hit 70 F. 5-10 days to germinate. From SFGate.

AnnualGermination
Temperature
(Fahrenheit)
LightingDays to
Germination
Weeks Sowing
to Planting
Ageratum (Ageratum houstonianum)70-75L7-108
Snapdragon (Antirrhinum majus)70L7-148-10
Wax Begonia (Begonia x semperflorens-cultorum)70-75L1410-12
Annual Aster (Callistephus chinensis)70L-D7-106-8
Vinca (Cathranthus roseus)70-75L-D1410
Cockscomb (Cleosia spp.)70-75D7-106-7
Bachelor’s Button (Centaurea cyanus)65-70L-D7-148
Cosmos (Cosmos spp.)70D5-74-6
Lisianthus (Eustoma grandiflorum)75L10-1414
Globe Amaranth (Gomphrena globosa)70L-D147-8
Sunflower* (Helianthus annuus)70D5-73-4
Strawflower (Helichrysum bracteatum)70-75L-D7-106-8
Impatiens (Impatiens wallerana)70-75L10-148-10
Annual Statice (Limonium sinuatum)70L-D7-108-10
Melampodium (Melampodium paludosum)65-70L-D7-107
Four-O’Clock (Mirabilis jalapa)70D5-76-8
Flowering Tobacco (Nicotiana alata)70-75L10-148
Geranium (Pelargonium x hortorum)70-75D7-2112
Petunia (Petunia x hybrida)75L7-108-10
Moss Rose (Portulaca grandiflora)75L7-1010
Black-Eyed Susan (Rudbeckia spp.)70L-D7-1410
Red Salvia (Salvia splendens)70-75L10-148
Mealycup Sage (Salvia farinacea)70-75L10-148-9
Creeping Zinnia (Sanvitalia procumbens)70D7-106-7
Coleus (Solenostemon spp.)70-75L10-148-10
Dahlberg Daisy (Thymophylla tenuiloba)65-70L148
Nasturtium* (Tropaeolum majus)65-70D10-145-6
Zinnia (Zinnia elegans)70D5-75

*Sunflowers and nasturtiums do not transplant well. Both should be seeded directly into peat pots.

For Zinnias – The germination process takes about 5-7 days at 75 degrees F. Remember, always keep the soil temperatures above 60 degrees F, ideally at greater than 70 degrees F. From here.

Light conditions during germination are critical for many annual flowers. The seeds of some plant species require light for germination. (In the table above, annuals that require light for germination are designated with the letter L in the lighting column.) After sowing these seeds, lightly press them into the germination medium, but do not cover them. The seeds of other flowers require darkness (D) and should be covered with the germination medium. Finally, those designated L-D should be lightly covered, leaving the seeds as close to the soil surface as possible.

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