Weathering The Elements
Nature’s peculiarities often alter
the growth and quality of homegrown vegetables.
Temperatures that are too high or too low, summer hail, and too much or
too little sun may damage crops. While
we can’t control Mother Nature, we can learn to co-operate or compensate so
that our gardens have a better than even chance.
Germination
Wisdom
Most vegetables germinate seed, grow,
and set fruit within a surprisingly narrow temperature range. Each vegetable seed has individual
requirements regarding soil temperature.
Lettuce and onion, for example, tolerate soil temperatures down to 32°
F; germination at such low temperatures is slow, but the seed will survive and
will sprout as the soil temperature warms up.
Bean and sweet corn seeds, on the other hand, will rot if left in the
ground more than a few days at temperatures below 55-60° F.
Vegetable seeds also cease
germinating when the temperature rises too high–somewhere between 86 and 104°
F, depending on the vegetable. During
hot weather, surface soil temperatures often climb far above this range. Even if the seed has germinated, the
seedlings (especially carrots and beets) can die of heat injury at the soil
surface. Gardeners can aid germination
and seedling survival during hot weather by spreading 2 inches of organic
material (such as compost) over the soil after the seeds have been planted. This protective layer reduces soil
temperature and holds in moisture.
Soil temperatures for Vegetable Seed
Germination
Vegetable Minimum Optimum Maximum
Asparagus 50 73 95
Beans (lima) 60 80 85
Beans (snap) 60 85 95
Beets 40 85 95
Broccoli 40 85 95
Cabbage 40 85 95
Carrots 40 80 95
Cauliflower 40 80 95
Celery 40 80 95
Cucumbers 60 95 105
Eggplant 60 85 95
Endive 32 75 75
Lettuce 32 75 75
Melons 60 95 105
Okra 60 95 105
Onions 32 80 95
Parsnips 32 70 85
Peas 40 75 85
Peppers 60 75 85
Pumpkins 60 95 105
Radishes 40 85 95
Spinach 32 70 75
Squash 60 95 105
Sweet corn 50 85 105
Swiss chard 40 85 95
Tomatoes 50 85 95
Turnips 40 85 105
To ensure good seed germination,
plant vegetables when the soil is within the proper temperature range. A soil thermometer is a handy tool to have. In addition, vegetable varieties have
individual temperature ranges. Early
varieties require a smaller amount of heat to mature than midseason varieties. Most miniature vegetables need less
cumulative heat than other vegetables.
If you live in an area of cool
summers (below 70° F), you can grow crops that require a longer or hotter
season (such as melons) by planting the earliest varieties. Those with the fewest days to maturity have the
lowest heat requirements. (Visit www.postagestampvegetablegardening.com for early varieties)
Fruit Set
Eggplant, peppers, tomatoes, beans,
and peas start to form fruit when the male parts of their flowers successfully
pollinate the female parts. The success
or failure of this pollination depends greatly on the temperature. The determining factor for such vegetables as
peppers and tomatoes is the nighttime, not the daytime temperatures. To set fruit, most tomatoes require
temperatures above 55° F for at least part of the night. Night temperatures above 75° F inhibit fruit
set and cause blossoms to drop.
Misshapen tomato fruit and puffiness
can result from high or low temperature extremes, which interfere with the
growth of pollen tubes and normal fertilization of the flower’s ovary. In addition, catfacing (puckering and
scarring of the blossom end of the fruit) can occur when cool weather at
flowering time makes the blossom stick to the small fruit. Many of these problems can be averted by
placing clear polyethylene covers over the plants when temperatures fall below
60° F.
©
Copyright by Karen Newcomb
