One of the most important factors for success with plants is the air. Yet, as it is not visible, it often goes unnoticed and is not considered so important. However it takes part in several different interactions with plants, and to understand them we need to examine what air is made of. The constituents that concern us are the carbon dioxide (which makes up a minute percentage), oxygen which is about a fifth, four fifths nitrogen and water vapor (or humidity). Each has a different role and each may affect growth.
Plants Need Carbon Dioxide
Carbon dioxide is converted by photosynthesis in most green parts (not just the leaves) into various organic chemicals which the plants then use for growth, and to store energy. Carbon is the key ingredient of all living matter and most of this is obtained from the air, although some is also taken up from the soil.
However as carbon dioxide is in such a small proportion, it can be rapidly removed from the air if this is not moving. In still air and bright sun a field of corn can use all available carbon dioxide within half an hour. Now, although there is a surplus of carbon dioxide in the whole system, this rapid removal down near ground level means that replenishment from elsewhere is necessary if plants are to keep growing. Fortunately almost all life in the soil is continually giving off carbon dioxide and this emerges underneath the leaves (where it is to some extent trapped) and is thus available to the plant.
Roots Need Air
Indeed, it is little understood how roots and all non-green parts of plants (as well most micro-organisms) breathe like us, taking in oxygen and giving out carbon dioxide. They are burning carbohydrates with oxygen for the energy to grow and perform other functions.
Because of this, soil needs to be aerated. A quick way to kill most plants is to waterlog the soil. Their roots drown as they can get no oxygen. Indeed the main benefit of improving drainage is to improve the aeration. Digging and incorporating gritty, fibrous or coarse textured materials may help but worms are the main aid as their tunnels provide easy ingress for air, down to two meters in some soils. (Also when heavy rain falls their tunnels fill, expelling stale air and then as the water drops down or is absorbed fresh air is sucked in.)
The Effect of Wind
Wind is one of the other more obvious effects of air and its effects can be good or bad. In many cases it causes direct damage, rocking plants and loosening roots, blowing off leaves and even breaking branches. It may also pick up small objects to batter the plants causing bruising.
Another major drawback of wind is its cooling effect. Although this may be welcome in hot weather, generally, in temperate climes, it acts to reduce the effective temperature and so slows growth. But even a wind as of just a few miles per hour causes most plants to reduce transpiration and growth. Strong cold dry winds are worst as they suck moisture from plants searing leaves and causing wind burn destroying leaves and flowers.
On the other hand in stagnant air, such as enclosed gardens with tall hedges or surrounding trees, the lack of air flow reduces growth, the likely high humidity causes an increase in disease (nothing promotes mildew, rusts and fungal diseases so effectively as plants in damp air with dry roots). Because the air is stagnant and not moving them then plants develop weak stems and become spindly and drawn, and probably etiolated (leggy, yellowish growth) as well if light levels are also lowered.
Increasing Throughput of Air
Where a garden is at the bottom of a slope then cold air, acting much like water, can pool and form a frost pocket. Enclosed gardens on slopes should have a gate, hole, gap, or some place that cold air can pour out. Never close off all openings on to a ditch or stream for this reason- it will be at a low point and is essential for removing cold air.
Under cover fresh air becomes even more important. Although plants do not like being in draughts they loathe stagnant air and all cover should be given maximum ventilation. Most greenhouses and tunnels are insufficiently well ventilated! One reason is that we like to keep them shut up for the warmth. Commercial growers do likewise but then supply carbon dioxide from compressed bottles. You can simply put fermenting liquids (sugar and yeast in water will do) under the staging to supply carbon dioxide in a similar way. (Or keep pets there – permaculture systems sometimes link a hen house with a greenhouse for this reason and for the added warmth.) Regular ventilation will still be necessary to prevent disease.
Finally, researchers using artificial light to grow plants found that over-crowding plants reduced their growth more from the hindrance to air movement than from the shading. So do remember to keep your plants well spaced.
By Bob Flowerdew.