It seems only a few years ago that John Gummer as Environment Minister was going on about how Global Warming would force us all to grow different crops in the future. In Cornwall they would be growing tea plants, we’d have cacti in our back gardens in Yorkshire, the staple crop of Scotland would be sunflowers and Norfolk would become one enormous paddy field – or something along those lines. If only global warming were as simple as that! Whilst he wasn’t completely wrong in his prediction (the Tregothnan Estate in Cornwall grows their own tea), the issue of Climate Change has not meant that the weather simply shifts.

In the mid-90’s I visited the British Antarctic Survey’s Headquarters in Cambridgeshire and they had a hundred years of data proving beyond any question of doubt, that the ice caps were melting, not just in fractional amounts as with seasonal adjustment, but drastically. That was 20 years ago, and it was evident to me that the increase in melted ice means more water.

One of the most critical factors in gardening is the presence or absence of water. No plant can live without it, being composed of about 90% water, but it is essential to get the amount of water a plant needs correct. The importance of water cannot be overstated but plants are as likely to die from too much water as too little. The climate is a critical factor in the life and livelihood of the horticulturalist. Everyone was aware of the impact last year’s “wettest drought on record”, but few people were more affected by it than those working from the land. Everyone in horticulture looking back at last year is not talking about the recession or the strength of the pound but the weather. And those looking forward (“what does 2013 hold for you?”) are saying the same: “it depends on the weather.” No matter how powerful man has become and how big the machines are, we are reliant on Mother Nature.

The increase in Global Warming, to return to my theme, has meant melting ice-caps. The cold water meets warmer water, creating air turbulence which means impact on the air temperature and more windy conditions. Increased water is evapo-transpirated into the atmosphere, the increased windflow blows it away from the point of origin and over Britain. And we don’t just get the rain we get freak heatwaves in March and strong gusting winds. The weather is not settled, it is in change and all that that implies, which is why we are getting freak weather. A few weeks ago we even had thunder and lightning during a snow storm. That is something I’ve never experienced before. Maybe in a few years or decades the weather will settle down some and we can get on with our outdoor cacti growing, but personally I doubt it. The ice-caps continue to shrink, Carbon Dioxide is decreasing and the planet is still overheating. So long as this doesn’t change we are likely to have a few years of unpredictability.

At its simplest level water stress in plants takes one of two forms.   Roots can get waterlogged and if the soil is anaerobic they will suffocate, if it is too dry they continue to loose water through their leaves and wilt. But both can happen at the same time. If we look at this in more detail, why does this happen? Plants ‘pull’ water out of the soil cell by cell. The pattern is that water flows from a weak solution to a stronger one through a semi-permiable membrane (the cell wall) by osmosis, across the cell by diffusion, then through the next cell by osmosis again all on up through the plant. This is called the transpiration stream and I learnt this at school and probably so did you. But what I never fully appreciated was why the pull? What is the end point of the strongest solution? It is the atmosphere, and plants can’t draw up water unless they also lose it. Plants are transpiring all the time, losing water through their leaves as long as the conditions are right. If the atmosphere is too cold or damp, it won’t happen so readily. The more water that plants pump into the atmosphere the less they can release into it. Some plants have adapted to this type of climate such as those of the cloud forest, where it is constantly humid, but for others it stops them drinking. They don’t need to keep their leaves from drying up, so they don’t transpire. Water is no longer pulled through the pathways of the plant and excess moisture stays, and accumulates, in the soil. Eventually the soil becomes waterlogged and the plant can’t breathe. But the chemical factories that produce chlorophyll and generate new cells can’t function without water, so they shut down. This leads to cell death and no matter if it’s hot, the plants can’t function and the root hairs which are only one cell thick and take up water can’t be replaced. This means they can’t drink, so more cells die. So the water can be wet and the plant can be lacking in moisture. This is called permanent wilting point and is why overwatered plants wilt, and if it reaches this stage it’s almost irreversible.

Excess moisture causes all sorts of other problems too, most notably an increase in fungal diseases. Fungi aren’t plants or animals, they belong to their own kingdom. When we think of fungi we envision mushrooms and toadstools, but these are just the fruiting bodies of the higher fungi (the equivalent of flowers). Most fungi are more primitive, often microscopic, organisms, and it is these that cause most plant diseases. We frequently don’t know much about them. They have many parts to their lifecycle, often some of the stages are yet to be observed by science. Sometimes different stages have different host plants, such as Wheat/Berberis Rust. They don’t need oxygen to survive, but what fungi need to spread their spores is moisture. Rain will cause pollen to rot, but not fungal spores! So it is no wonder there is a prevalence of ‘new’ diseases striking the horticultural world.

And pests are another issue. While some don’t do well in ‘summers’ like the one we have just had, it has been a boon year for the gastropod. The advice of gardening journalists a few years ago was that we should buy drought tolerant plants. This is clearly not the answer. That remains elusive while the weather is still fluctuating. I feel the answer is to plant for the short term, hope for the best and expect the worst.   At least it will keep our struggling nurserymen in business if we have to replant every few years. Personally, I’m just hoping for a few weeks of decent hot weather this year, preferably while I’m on holiday!

 

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