Different vegetables have different germination requirements. It’s important to know the germination requirements of the seeds you are planting to ensure success.
Viable seeds are alive; they contain an embryo, stored food and are surrounded by a seed coat. A seed will germinate or begin to grow when soil moisture, temperature, and oxygen as well as light conditions are right for that specific seed.
Nearly all vegetable seeds will germinate at about 60° to 70°F (16-21°C) if they are in contact with moist, aerated soil; the soil should be moist to about 50 to 75 percent of the soil’s moisture holding capacity–that is moist not wet. Many vegetable seeds—generally cool-weather crops—will germinate in soil as cool as 45° to 50°F (7-10°C) —a few in even cooler soil.
Some vegetable seeds have special requirements to germinate. They are protected by seed dormancy—that is they require special circumstances beyond what we might consider normal germination requirements. Seed dormancy is genetic and the product of a specific plant species adaptation to its natural environment over thousands of years.
Normal Vegetable Seed Germination Requirements
All seeds should be planted in fine-textured soil. That is humus rich loamy soil that contains well-decomposed organic matter. Fine-textured soil is well aerated which allows for good gas exchanged between the embryo of the germinating seed and the soil. (Seeds like people need oxygen; oxygen is partially converted to carbon dioxide during growth; the carbon dioxide needs to move away from the seed through aerated soil.)
Soil for seed starting should be just moist; moisture is needed by most seed for the seed coat to swell and open; water is essential for seeds and plants to transport nutrients throughout the seed and plant.
Most vegetable seeds should be planted to a depth that is 3 times the thickness of the seed. For example, a bean seed as large as 1 inch wide should be planted 3 inches deep. A carrot seed which is ¼ inch wide should be planted ½ inch deep.
Exceptions to Normal Germination Requirements
Some vegetable seeds have special germination requirements. For seed sowing success, here are exceptions you should know:
Seeds that needs light to germinate:
Some vegetable seeds—mostly very small ones—require light–not darkness–to germinate. The seeds from these plants are usually very small and naturally drop from spent flowers or drop to the soil in rotting fruits.
Seeds that require light to germinate are lettuce and savory. These seeds should not be covered with soil or just barely. Press these seeds into the soil with you finger or thumb so that the seed is just surrounded but not covered with soil. Keep the soil just moist and protect these seeds from drying sun, wind, and foraging birds by covering the seed bed with a light floating row cover.
Seeds that need little soil cover to germinate are: broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, collards, cucumbers, eggplants, kale, kohlrabi, leeks, melons, peppers, squashes, and tomatoes. These seeds can germinate in light if surrounded by moist soil or they can be lightly covered with soil.
Seeds that benefit from soaking in water:
Some seeds germinate quicker if they are soaked in water or water and a drop of vinegar before they are planted. Seeds with hard coats, big seeds, and seeds with wrinkled coats benefit from soaking in water before planting.
These seeds include asparagus, beans, carrots, corn, okra, parsley, peas, pumpkins, and squash. Beet seeds and Swiss chard seeds will benefit from soaking in water also.
Put these seeds in a shallow bowl of water or place them on a damp paper towel and put them in a sealed plastic bag to soak up the moisture. Warm water will reduce the soaking time. Add a few drops of kitchen vinegar to the water; the acid in the vinegar will also help breakdown the seed coat.
Soak these seeds just long enough for the seed coats to draw up moisture—about 12 hours or overnight.
Seeds that benefit from being scratched or nicked:
Seeds with very hard coats will germinate more quickly if the seed coat is scratched or nicked and then soaked in water before planting. This process is called scarification. Seeds that can benefit from light scratching before planting include beans, melons, and squashes.
Scarification should weaken the seed coat so that moisture is more easily soaked up. Do not puncture the seed coat; this can kill the seed embryo.
Seeds that germinate in cool soil:
Most cool-weather vegetables have naturally adapted to germinate best in cool not very warm soil. Seeds best planted early in cool soil include: arugula, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, Chinese cabbage, collards, garden cress, kale, kohlrabi, leeks, lettuce, mustard, onions, parsley, parsnip, peas, radish, spinach, and turnips.
Herbs that prefer to be planted early in cool soil include borage, chamomile, chervil, coriander, dill, marjoram, and thyme.
Seedlings that resent transplanting:
The roots of some seedlings do not take well to transplanting.
Vegetables and herbs best started in place in the garden include: anise, arugula, borage, carrots, chervil, coriander, dill, fennel, mustard, parsley, parsnips, peas, radish, rutabaga, spinach, Swiss chard, and turnips.
If you start these crops indoors and transplant them out to the garden, just nip off the tip of the root before transplanting. Tipping the end of the root will allow long roots to fit in the planting hole.