How temperature affects plant growth

Average day-to-day temperatures play an important part in how your vegetables grow. Temperatures, both high and low, affect growth, flowering, pollination, and the development of fruits. If the temperature is too high or too low, leafy crops may be forced to flower prematurely without producing the desired edible foliage. This early flowering is called "going to seed," and affects crops like cabbages and lettuce. If the night temperatures get too cool it may cause fruiting crops to drop their flowers — reducing yields considerably; peppers may react this way to cold weather. Generally, the ideal temperatures for vegetable plant growth are between 40° and 85°F. At warmer temperatures the plant's growth will increase, but this growth may not be sound structural growth. At lower temperatures the plant's growth will slow down or stop altogether.

Vegetables have different temperature preferences and tolerances and are usually classified as either cool-season crops or warm-season crops. Cool-season crops are those like cabbages, lettuce, and peas, which must have time to mature before the weather gets too warm; otherwise they will wilt, die, or go to seed prematurely. These vegetables can be started in warm weather only if there will be a long enough stretch of cool weather in the fall to allow the crop to mature before the first freeze. Warm-season crops are those vegetables that can't tolerate frost, like peppers, cucumbers, and melons. If the weather gets too cool they may not grow at all; if they do grow, yields will be reduced. Warm-season crops often have larger plants than cool-season crops and have larger, deeper root systems that enable them to go for relatively longer periods without being watered. Even though it is convenient to think of vegetables simply as either cool-season or warm-season crops, considerable differences can exist within each of these two groups.

The following lists offer a guide to cool- and warm-season crops. For specific planting dates for each type of vegetable, refer to the chart at the end of "Planting Your Garden."

Cool-season vegetables include: globe artichokes, asparagus, beets, broad beans, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celeriac, celery, chard, chicory, Chinese cabbage, collards, cress, dandelion, endive, cardoon, horseradish, Jerusalem artichokes, kale, kohlrabi, leeks, lentils, lettuce, onions, parsnips, sweet peas, white potatoes, radishes, rhubarb, rutabagas, salsify, shallots, sorrel, spinach, and turnips. Cool-season herbs include: anise, borage, chive, dill, oregano, parsley, peppermint, rosemary, sage, savory, spearmint, tarragon, and thyme.

Included among the warm-season vegetables are: dry beans, lima beans, mung beans, snap or green beans, chayote, chick peas, corn, cucumbers, eggplant, muskmelons, mustard, okra, black-eyed peas, peanuts, peppers, sweet potatoes, pumpkins, soybeans. New Zealand spinach, summer squash, winter squash, tomatoes, and watermelons. Warm-season herbs include: basil, caraway, chervil, coriander, marjoram, and sesame.

Rainfall: How plants use water

The amount and timing of the rainfall in your area also affects how your vegetables grow. Too much rain at one time can wash away seeds or young seedlings and damage or even kill mature plants. A constant rain when certain plants are flowering can reduce the pollination of the flowers and reduce yields. This can happen to tomatoes, peppers, beans, eggplant, melons, pumpkins, and both summer and winter squash. A constant rain can also tempt the honeybees to stay in their hives instead of pollinating the plants; again, yields will be affected.

Too little rain over a period of time can slow down plant growth and kill young seedlings or even mature plants. Limited moisture in the air can also inhibit pollination and reduce the yields of some vegetables. Too little rain can be more easily remedied than too much. If it rains too little, you can water the garden. If it rains too much, all you can do is pray.

Rainfall is probably the easiest climatic condition to improve. Farmers have worried and complained about the rainfall since the beginning of agriculture. If you've got thousands of acres of land and no control over the available water it can be very frustrating — if not a disaster. Since the home garden is usually small and fairly manageable in size, you can do something to regulate how much water it gets. If you don't get enough rain when you need it, you can simply water, and there are many different methods you can use. These are described in detail in "Caring for Your Garden." Too much rain can be more difficult to deal with, and here you need to take preventive measures. The better drained your soil is, the better it will be able to deal with too much water. When you select the site for your garden, avoid any area that is low-lying or poorly drained. If that's the only site that you have for the garden — and you're really serious about gardening — you can improve it by installing drainage tiles. This can be a costly and complicated process, so consider it only as a last resort.

Light: Your plants can't live without it

The third major climatic factor is light, and it's an important factor to consider when you plan your garden. Sunlight — or some type of light — provides energy that turns water and carbon dioxide into the sugar that plants use for food. Green plants use sugar to form new cells, to thicken existing cell walls, and to develop flowers and fruit. The more intense the light, the more effective it is. Light intensity, undiminished by obstructions, is greater in the summer than in the winter, and greater in areas where the days are sunny and bright than in areas where it's cloudy, hazy, or foggy. As a rule, the greater the light intensity the greater the plants' production of sugar — provided, of course, that it's not too hot or too cold and the plants get the right amount of water.

If a plant is going to produce flowers and fruit, it must have a store of energy beyond what it needs just to grow stems and leaves. If the light is limited, even a plant that looks green and healthy may never produce flowers or fruit. This can be a problem with vegetables like tomatoes, where you want to eat the fruit. With lettuce, where you're only interested in the leaves, it's not an issue. All the same, all vegetables need a certain amount of light in order to grow properly, and without it all the watering, weeding, and wishing in the world will not make them flourish.

How day length affects your crops. Many plants, including tomatoes and many weeds, are not affected by day length — how long it stays light during the day. But for many others the length of the day plays a big part in regulating when they mature and flower. Some plants are long-day plants, which means they need 12 or more hours of sunlight daily in order to initiate flowering. Radishes and spinach are long-day plants, and this is the main reason they go to seed so fast in the middle of the summer when the day length is more than 12 hours. If you want to grow radishes or spinach in midsummer, you have to cover them with a light-proof box at about 4 p.m. every afternoon to fool them into thinking the day's over. Other plants are short-day plants and need less than 12 hours of light to initiate flowering; soybeans and corn are examples. Many varieties of short-day plants have been bred to resist the effects of long days, but-most will still flower more quickly when the days are shorter.

How much sunlight is necessary? Vegetables grown for their fruits need a minimum of six to eight hours of direct light each day. Less light frequently means less than a full crop. It's very frustrating to try to grow tomatoes, peppers, or eggplants in the shade; they'll often produce a good, green plant without giving you anything at all in the way of a vegetable. Crops that are grown for their roots and leaves, however, will give you satisfactory results in light shade.

Root crops, such as beets, carrots, radishes, and turnips, store up energy before they flower and do rather well in partial shade, especially if you don't compare them with the same crop grown in full sun. Plants like lettuce and spinach that are grown for their leaves are most tolerant of shade; in fact, where the sun is very hot and bright they may need some shade for protection. Only mushrooms and sprouts can be produced without any light at all.

Making the most of your garden light. If you have a choice of where to grow your vegetable garden, don't put it in the shade of buildings, trees, or shrubs. The accompanying illustration shows how to give plants enough light. Remember that as well as shading an area, trees and shrubs also have roots that may extend underground well beyond the overhead reach of their branches. These roots will compete with the vegetable plants for nutrients. Stay clear especially

Scientific Walnut Growing

of walnut trees; they produce iodine, a growth retardant that will stunt or kill the vegetable plants in your garden. Go out and stand in your garden to see just how the light falls. Walk around and find where the light fails to penetrate. This knowledge will be very useful when you come to planting time.

Providing shade from too much sun. Most vegetables need full sun for best growth, but young or newly transplanted plants may need some protection from bright, direct sunlight. It's easier for you, as a gardener, to provide shade where there's too much sun than to brighten up a shady area. You can, for instance, plant large, sturdy plants like sunflowers or Jerusalem artichokes to provide a screen, and you can design your garden so that large plants and small ones each get the light they need. You can also shade young plants with boxes or screens when necessary. However, too little sun is far more serious a problem in a garden than too much.

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