Are you getting ready to sow vegetable and herb seeds indoors? It can be very handy to know what temperatures they prefer for best germination. That way, you’ll know whether to place the seed tray in a warm spot (or even provide bottom heat) or instead, if that particular plant prefers things cooler, another location where you can keep temperatures down.
Fast forward several weeks and you’ll now be thinking about those vegetables and herbs that grow best when started outdoors directly in the garden. Yet some of them like fairly cool soil and need to be planted outdoors shortly after frost is out of the ground, faltering if things heat up too much too quickly, while others may rot or remain stunted and unresponsive if you sow them too early, when the soil is still cold. What are you to do?
First of all, you’ll find the seed chart below very handy. It shows the preferred germination temperature for over 50 commonly grown vegetables and herbs.
However, you’ll also need to know where the warm and cool spots in your home are located (for indoor seed starting) and what the soil temperature is in your outdoor vegetable garden. For that, you’ll need a thermometer … in fact, two thermometers.
An electronic thermometer is ideal for testing indoor air temperatures. Most will check both the maximum temperature (usually during the day) and the minimum temperature (at night), so leave them in place for 24 hours for a complete picture. Temperatures inside a covered seed tray will be closer to the maximum temperature reading and, unless the spot cools off considerably at night, will remain quite stable, a further factor to consider.
A soil thermometer or compost thermometer—much like a meat thermometer, but more rugged—is best for measuring outdoor soil temperatures. Just plunge it into the soil to a depth of about 2 to 4 inches (5-10 cm) either in a shady spot or shade it with your hand (never take readings in the burning sun; you’ll get false results) and wait a minute or so for the reading. Don’t worry about lower night-time temperatures unless you’re expecting a major and long-lasting cold front: soil temperatures underground are quite stable, warming up ever so slowly as spring progresses, but not dropping much in cool weather.
Seed Germination Chart
The chart below offers the optimum temperature for germination of over 50 vegetables and herbs. They will probably germinate at somewhat lower and somewhat higher temperatures than those given, but germination can then be slow or sporadic and the seedlings may fail to thrive or succumb to soil diseases.
|Artichoke (Cynara cardunculus)||70-75 °F (21-24 °C)|
|Asparagus (Asparagus officinalis)||60-85 °F (15-30 °C)|
|Basil (Ocimum basilicum and others)||65-85 °F (18-30 °C)|
|Bean (Phaseolus vulgaris)||60-85 °F (15-30 °C)|
|Beet or Beetroot (Beta vulgaris Condivita group)||50-85 °F (10-30 °C)|
|Broccoli (Brassica oleracea italica)||45-85 °F (7-30 °C)|
|Cabbage (Brassica oleracea capitata)||45-95 °F (7-35 °C)|
|Carrot (Daucus carota)||45-85 °F (7-30 °C)|
|Cauliflower (Brassica oleracea botrytis)||45-85 °F (7-30 °C)|
|Chard, Swiss (Beta vulgaris Flavescens Group)||50-85 °F (10-30 °C)|
|Chervil (Anthriscus cerefolium)||60-65 °F (15-18 °C)|
|Chicory or Escarole (Chichorum intybus)||65-75 °F (18-24 °C)|
|Chinese Cabbage (Brassica rapa pekinensis)||50-80 °F (10-27 °C)|
|Chives (Allium schoenoprasum)||60-70 °F (15-21 °C)|
|Coriander or Cilantro (Coriandrum sativum)||55-70 °F (13-21 °C)|
|Corn or Maize (Zea mays)||60-95 °F (15-35 °C)|
|Cucumber (Cucumis sativus)||60-95 °F (15-35 °C)|
|Dill (Anethum graveolens)||60-70 °F (15-21 °C)|
|Eggplant or Aubergine (Solanum melongena)||75-90 °F (24-32 °C)|
|Endive (Chichorium endivia)||55-70 °F (13-21 °C)|
|Fennel (Foeniculum vulgare)||60-70 °F (15-21 °C)|
|Ground cherry (Physalis pruinosa)||65-85 °F (18-30 °C)|
|Kale (Brassica oleracea acephala)||45-75 °F (7-24 °C)|
|Kohlrabi (Brassica oleracea gongylodes)||40-90 °F (5-33 °C)|
|Lavander (Lavandula angustifolia)||65-70 °F (18-21 °C)|
|Leek (Allium porrum)||65-75 °F (18-24 °C)|
|Lettuce (Lactuca sativus)||40-80 °F (5-27 °C)|
|Majorum (Origanum majorana)||60-70 °F (15-21 °C)|
|Melon (Cucumis melo)||75-90 °F (24-35 °C)|
|Mint (Mentha spp.)||68-75 °F (20-24 °C)|
|Mizuna (Brassica juncea japonica)||45-85 °F (7-30 °C)|
|Okra or Gumbo (Abelmochus esculentus)||70-95 °F (21-35 °C)|
|Onion (Allium cepa)||50-95 °F (10-35 °C)|
|Oregano (Origanum vulgare)||72-77 °F (22-25 °C)|
|Parsley (Petroselinum crispum)||50-85 °F (10-30 °C)|
|Parsnip (Pastinaca sativa)||50-70 °F (10-21 °C)|
|Pea (Pisum sativum)||40-75 °F (5-24 °C)|
|Pepper (Capsicum annuum)||60-95 °F (15-35 °C)|
|Pumpkin (Cucurbita pepo)||70-90 °F (21-32 °C)|
|Pumpkin (Cucurbita pepo)||70-90 °F (21-32 °C)|
|Radish (Raphanus sativus)||45-90 °F (7-32 °C)|
|Rosemary (Rosmarinus officinalis)||80-90 °F (27-32 °C)|
|Rutabaga or Swede Turnip (Brassica napobrassica)||45-85 °F (7-30 °C)|
|Sage, Common (Salvia officinalis)||60-70 °F (15-21 °C)|
|Savory, Summer (Satureja hortensis)||60-70 °F (15-21 °C)|
|Spinach (Spinacia oleracea)||45-75 °F (7-24 °C)|
|Spinach, New Zealand (Tetragona expansa)||60-75 °F (15-24 °C)|
|Squash (Cucurbita pepo and others)||70-95 °F (21-35 °C)|
|Thyme (Thymus vulgaris and others)||60-70 °F (15-21 °C)|
|Tomatillo (Physalis ixocarpa and P. philadelphica)||60-95 °F (15-35 °C)|
|Tomato (Solanum lycopersicum)||60-85 °F (15-30 °C)|
|Turnip (Brassica rapa rapifera)||60-105 °F (15-41 °C)|
|Watermelon (Citrullus lanatus)||70-95 °F (21-35 °C)|
|Zucchini (Cucurbita pepo)||70-95 °F (21-35 °C)|
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are there any vegetable seeds that specificately need bottom heat
Absolutely need it, no. Generations of gardeners selecting for easy to grow plants has largely eliminated that. But the solanums (tomatoes, peppers, eggplants, etc.) do love a heat boost early one, as does okra.
Very good close to what I have found over the 59 years I’ve played in the dirt.