Masterclass: Positive Stress?, Can being “unkind” to your plants ever be a good thing?
Stop reading this. Please. Make sure you’ve completely dialed in your grow first. It’s far more important! Perfect your daytime and nighttime temps, keep a tight grip on your relative humidity, maintain optimum light levels, exact your feeding regimen, the works – all of it, get it right first.
The techniques described in this article are NOT for beginners and some of this stuff is nothing short of contentious. (Hmmm, a sure way to peak your interest though, eh?) We’re going to discuss methods of taking your plants to their outer limits and making them go a little bit crazy in the process. So, if you’re coming with us on this journey, buckle up, put on your questioning hat, and hold on to it tightly! Here we go …
Personally, I don’t care for stress, especially that special kind imposed by magazine publication deadlines. But I have to admit, one positive effect these deadlines have is to make me work harder!
So what about plants? We all know that unfavorable growing conditions (e.g. high temperatures and low relative humidity) can stress plants out big time. And if these conditions are severe enough you’re the quality and quantity of your yields will suffer. Invariably badly stressed plants will yield less than plants that have been pampered in every way.
Similar to most things in life, plants stress is not as black and white as you may think. In biology, stress can actually strengthen an organism. Immunity is obtained only from being subjected to an infection which involves suffering followed by growth, resistance and strength. In the human body a muscle cannot grow without being subjected to stress; a broken bone, when it sets properly, binds stronger than it was before and for that reason is very unlikely to break a second time at the same juncture. I’m sure many of you will have heard the phrase “What doesn’t kill you makes you stronger” or “Treat ‘em mean, keep ‘em keen”? Well to a certain degree there is some truth in these sayings.
So with all this in mind is it at all possible that some forms of mild plant stress can actually improve results? The idea that stress can actually be a positive thing may seem alien to some of you. Why would you purposely want to stress plants? Surely stress should be avoided at all costs, right? Well the answer is “yes” and “no.” Here we explore the realms of stress; types that should be avoided and others which may well help you push your plants from being lazy and complacent into restless and eager producers!
There are many factors that will cause plants to become stressed, most of which can be grouped into two general categories –
Physical / Mechanical Stress:
Manipulation – Bending or training stems, physically damaging the plant.
Pruning – Removing leaves, stems, flowers or fruits.
Denial – not allowing a certain physical growth factor; e.g. blocking light, preventing pollination
Environmental / Abiotic Stress:
Water – drought, over-watering.
Temperature and Humidity – Cold (chilling and freezing), heat, wet or dry.
Mineral deficiency or toxicity – incorrect fertilization or salinity.
Pests and disease.
High heat in your indoor garden can create a myriad of problems, the most common of which are tall leggy plants with large intermodal spacing, small fruits and loose flowers, high water usage, lower nutrient tolerance, and if temperatures remain high for long periods the stomata will close, plant growth will slow right down and may even cause severe wilting. Low temperatures are less problematic for indoor growers but can occur and cause slow growth and poor nutrient uptake.
Low humidity during hot weather is a common problem that should be monitored and avoided as it will cause elevated transpiration and high water usage, increased susceptibility to over fertilization, leaf roll, stomata closure, and stunting. Periods of very low humidity can also cause wilting. On the flip side, high humidity will invite fungal infection to take hold and will slow the uptake and transport of water and nutrients.
Consistently irrigating the root zone beyond the plant’s usage capability causes a depletion in oxygen. These anaerobic conditions create a poor environment for root growth, cause poor water and nutrient uptake and favors the development of root diseases. A persistent lack of moisture around the rhizosphere can cause wilting, weakened leaf tissue, permanent root damage and nutrient precipitation in the growing media.
Having the grow lights too close will cause localized high heat and low humidity, this will lead to elevated transpiration and may result in permanent leaf tissue damage. Not having enough light tends to create poor plant growth as low water and nutrient uptake occurs. It most commonly causes elongated stems and large intermodal spacing.
All good growers understand the importance of their nutrient solution’s pH and keep it within the range of 5.5-6.5, thus allowing nutrients to be available for uptake. If the pH swings out of this range for prolonged periods then nutrients that your plants need will be unavailable or ‘locked out’ which will eventually lead to mineral deficiencies
High nutrient levels can cause permanent damage to your plants. Symptoms include tough leathery foliage, very dark green growth, leaf curl, poor water uptake and leaf tissue necrosis (death). Low nutrient strength is not so damaging but can also create unwanted characteristics such as soft weak stems and leaf tissue, mineral deficiencies, leggy growth and poor fruit and flower development.
Pests and Disease
Aside from pests physically eating the leaves and causing direct tissue damage, they can also spread disease from plant to plant or make a plant more susceptible to diseases and infections. Plants can usually recover from pest attack if the problem is dealt with quickly but diseases are a little more tricky. Above ground, fungal diseases like powdery mildew or botrytis can be controlled once they have infected the plant but root pathogens, viruses and other forms of invasive diseases are difficult, if not impossible to shift once they have taken hold.
Heard all this before? Okay, well now it’s time to introduce some more concepts that may not be so familiar…
It’s good practice to do all you can for your plants during propagation and their early growth stages because keeping plants healthy during this time is crucial when creating healthy vigorous plants. As plants mature and start producing fruits or flowers, small amounts of stress applied in the right way can actually help to improve the plant’s favorable characteristic. This may be an enhanced flavor, early ripening, elevated resistance to disease or enhanced chemical/medicinal characteristics.
Positive stress techniques are often used in commercial horticulture as a tool for influencing or ‘steering’ plants into a growth habit which the grower desires. Steering plants with mild stresses can influence the plant into shifting its efforts from vegetative growth into fruit or flower production.
Sometimes you want your plants to grow, sometimes you want them to bloom. Most growers are familiar with using their light cycles to steer photoperiod sensitive plants. But this is just the tip of the iceberg. For most cultivated fast growing plants, “mild conditions” play a large role in steering the plant towards a vegetative growth habit. Here are some examples:
Lower nutrient strengths
The idea here is that by using a low nutrient strength you makes it easy for plant to take up water and nutrients through the roots. Obviously, you need to supply enough nutrients so as not to cause any deficiencies or unwanted growth characteristics (stretching/ long internodal distance), so supplying your plants with just above the minimum to reach these requirements will make it ‘easy’ for the plant, less stressful and therefore help keep the plant vegetative. I typically feed at a EC of 1.2 throughout veg and flower.
Wetter Root Zones
By regularly replenishing the growing media or root zone with water and nutrients without allowing dry periods to occur, the grower allows the plant easy access to water and nutrients, helping to steer the plant in a vegetative direction. It’s not good practice to purposefully over water the root zone in an attempt to steer vegetatively – all this will do it drive out vital oxygen and impede root function. The aim is to understand your plants’ (and growing media’s) water requirements and irrigate just before the growing media starts to dry. To implement this technique, drip irrigation systems offer most control. The irrigation strategy employed should be short irrigations with a high frequency. These irrigations should supply a little more than the amount the plants are using, with only a small amount of runoff occurring. By allowing water and nutrients to be constantly available to the plant it minimizes stress and promotes a vegetative growth habit.
Warmer Root Zones
Heating the nutrient solution will make it easy for the roots to function and take up water and nutrients easily. Aiming for 70°F (21°C) will help to make it easy for the plant and steer towards vegetative growth.
Low “Dif” – Small Difference in Day and Night Temperatures
The difference between the maximum daytime temperature and minimum nighttime temperature is often referred to as the ‘dif’, and contributes significantly towards your plants’ state of growth. By keeping the dif as small as possible the grower stimulates vegetative growth and keeps the plants short and compact. This is a really crucial technique for all indoor growers to get their heads around as shorter plants tend to yield far more under grow lights. Ideally, to keep plants vegetative and squat you should aim for a dif of no greater than 7°F (4°C). Time to buy that block heater!
Mild Environmental Conditions
To steer plants vegetatively, it’s important that the environment is as stress free as possible; therefore efficient temperature and humidity control are vital. Stress free growing conditions will be created if plants are able to transpire comfortably and create assimilates (sugars) via photosynthesis effectively. This will be achieved if the air temperature and humidity is within the plant’s comfort zone, generally 60-70% relative humidity (RH) with the air temperature between 68-77°F (20-25°C) – these conditions should make it comfortable for the plant to function vegetatively .
Encouraging plants to flower quickly is a key skill for every indoor grower to acquire. The last thing any of us want are tall, stretching, leggy plants that force us to raise up our grow lights. To get the most out of grow lights indoor gardeners aim for shorter, compact plants with wide canopies – the best way to harness as much of that precious incident light energy from grow lights as possible. To influence the plant’s speedy shift from a vegetative growth into flower or fruit production (generative growth), most indoor growers cultivating photosensitive plants will alter the light cycle and change out the nutrient solution from a ‘grow’ formula to ‘bloom’, and maybe use a few blooming additives through the cycle. This may meet the plant’s basic requirements to start producing fruit or flowers, but selectively using mild stresses can not only trigger your plants into generative growth more quickly and efficiently, but it can also help focus your plants, throughout the flowering stage, to drive their efforts into producing copious amounts of flowers and fruits. These generative steering tools include:
Higher Nutrient Strengths
By raising the strength of the nutrient solution you are effectively increasing the concentration of mineral salts around the roots. This situation makes it more difficult for the plant to uptake water. When carefully managed, raising the nutrient strength to just below your plant’s upper tolerance will create a mild stress around the roots and steer the plant towards generative growth. Before undertaking this measure it is important you know your plant’s nutrient tolerance, some species and even different varieties within species will be able to tolerate more nutrient than others. Most importantly, you must have good environmental control to implement this stress technique. If you have problems with low relative humidity (below 50%) or do not have good temperature control, I strongly advise against using high nutrient strength as a steering tool as you will most likely cause problems with over feeding. Only with optimum environmental control can you accomplish generative steering with elevated nutrient strength.
Drier Root Zones
Allowing the growing media to dry slightly between irrigations also causes mild stress. The aim is not to completely restrict the availability of water and certainly not to allow the plant to wilt. The goal is to allow the growing media to dry to a point where the roots are ‘worried’ that water is running out, but not so much as to allow complete dehydration of the root surface. Implementation of this is fairly simple, during veg you water little and often to stimulate vegetative growth so during flower you water larger volumes less frequently. You don’t have to alter the total volume of water given during a day, just the timing of the irrigations. Once aging this technique is most controllable with drip irrigation systems.
As well as the frequency of the irrigations, the start and stop time can also be used as a steering tool. During the night your plants still use small amounts of water, this creates a drying back of the growing media during the night cycle. The more the growing media dries overnight, the more of a generative action it has. If the growing media is not drying much during the night it may be because you are irrigating too close to the lights turning off, which is more suited to vegetative growth. Your chosen time to stop and start irrigations will be determined by the growing environment but generally, starting one hour after the lights come on and one hour before they go out will be a good base to start from. If you hand water your plants in veg, say 35 fluid ounces (just over a liter) each day and you get a small amount of runoff, you could change to watering 70 fluid ounces every two days to make your plants more generative.
Word of warning; if the growing media dries too much and does not receive enough nutrient solution to re-saturate it, the nutrient strength in the growing media will start to rise. This will add to steer the plant generatively, but may lead to over fertilization. Always ensure that during the peak irrigations you supply enough solution to re-saturate the growing media and achieve 10-20% runoff.
Colder Root Zones
If you have some degree of control over the temperature of the nutrient solution you can stimulate generative growth by slightly cooling the solution. A drop from 70°F (21°C) down to 65°F (18°C) will make it slightly more difficult for the roots to function, but they will still be more than able to take up water and nutrients effectively. This mild root zone stress will not harm growth, it will just nag at the plant and push it in a generative direction.
Larger Difference in Day and Night Temperatures
Increasing the dif is known to have a positive generative action for most cultivated plants. However, it is not always good for plants that grow large fruits or flowers for the temperature to drop much below 65°F (18°C) as the transportation of assimilates made during the day can be affected by cold nights. If you have moderate day temperatures (75°F/24°C) and cannot achieve your desired dif, you may find it beneficial to raise the day temperatures to enough to keep the plants growing healthily (80°F/26.5°C) in order to enable you to increase the daily dif. To steer plants generatively try to aim for a dif of around 15°F (8°C).
Rapid late evening temperature drops have been used by greenhouse tomato growers for many years as a way of forcing assimilates towards the fruits. A quick fall in air temperature causes the plant to also cool down, but the leaves cool much faster than the fruit. This difference in internal temperature causes a draw of photosynthetic assimilates from the large leaves, which have been working hard to make sugars throughout the day, to be translocated to the fruits to advance growth. These quick pre-night temperature drops are not so difficult to achieve in an indoor garden because when the grow lights switch off, temperature often drops quickly. As long as the temperature falls enough to quickly cool the leaves, and is them maintained at a reduced level, the fruits or flowers can often stay warmer than the leaves for more than an hour. This maybe a mild stress technique you are already employing without realizing it!
Slightly Harsh Environmental Conditions (warmer temperature, lower RH)
One of the most severe stresses you can inflict on a plant is environmental stress. High temperatures coupled with low RH will make it near impossible for most cultivated plants to grow successfully. However, if you have optimum environmental control systems in place, a slight increase in day time temperature combined with a slight decrease in RH can have a significant impact. If, for example, during vegetative growth you are maintaining the day time temperature at 75°F (24°C) with 70% RH, a slight increase for a few hours each day to 80°F (26.5°C) while maintaining the same RH will increase plant metabolism and transpiration rates for short periods creating small periods of mild stress. During these periods the plant is still fairly comfortable and able to function properly but these slightly harsher conditions steers the plant more in the direction of generative growth.
Elevated CO2 levels
Higher CO2 levels in the growing environment increases photosynthetic rate. This in turn creates and provides more assimilates to the developing fruits and flowers. This means better fruit and flower initiation and overall more fruit or flowers on the plant which guides the plant in a generative direction. Dosing CO2 early in the light cycle will have a more generative action as this is when peak growth occurs.
Crop steering techniques are great tools to have in your arsenal when trying to get the most from your plants. When growing vine (indeterminate) tomato varieties or sweet and chili peppers you want to harvest fruits for as much time as possible, generative or vegetative steering techniques can be used to balance the plants into a state of constant production e.g. not too vegetative and not too generative.
When growing short cycle plants like bush (determinate) tomato plants or flowering annuals, the goal is to push the plant from a vegetative direction and force it into a generative state and keep it as generative as possible. This will result in one big flush or fruits or flowers to be harvested in one go. Generative steering techniques are extremely valuable when used in the later stages of a short cycle plants life to help it drive all its efforts into generative production.
So, mild stress can be used keep plants growing in your desired direction, but what other stress techniques are out there that may be able to help us achieve better results?
There are many myths that circulate in grower circles about techniques to enhance quality and quantity. One that I would like to quash before we go any further is the stress technique of inserting a nail through the base of the stem. Many times I have heard “This old grower I know says hammering a nail through the stem just before the end of the plant cycle makes the final fruits smell and taste better”. Total BS! Hammering anything into your plant is a sure fire way to majorly stress it out, potentially reduce yield considerably and result in very little change to your end produce. May this myth die a horrible death, much like the hammered plants will.
Topping is a technique that most growers are familiar with to transform a tall skinny plant into a short, wide bush. Removing the growing tip reduces ‘apical dominance’ which is where the central stem is dominant over other side branches. By removing the growing tip early the plant’s life, many side shoots grow which helps indoor growers create a more even canopy when growing under lights. Removing the growing tip causes some considerable stress to the plant but creates much more productive and controllable plants in the long run.
Thinning or complete clearing of bottom growth is a technique often utilized when growing indoors to concentrate the plant’s efforts into producing good quality fruits and flowers that are bathed in light. Stems, leaves and flowering sites that are in complete shade will end up producing very little, so removing them may cause some initial stress in the short term but the plants will benefit from more concentrated growth in the long term. Many growers say that the thinning process, when done in the early stages of the flowering cycle helps to speed up the onset of flower. It may be that the removal of plant material stresses the plant in a generative direction. Fruit thinning is often carried out when growing cucumbers, peppers and even apples. Removal of some of the smaller fruits helps divert energy toward the larger fruits and results in better quality large fruits rather than lots of small ones.
When growing sub terrain crops like garlic and potatoes, growers want the plants to put all their efforts into producing those underground delights. To help them do this growers stress the plants by removing the flowering stems when they appear in midsummer. This stops the plants investing energy in fertilizing their flowers and producing seed, and focuses their attention into producing large tubers or bulbs. The picture below show the effect of removing the flowering stalk, aka ‘scape’, from the garlic plants compared to leaving them on. Significant increases in yield can be made using this technique.
Air pruning roots is another great example of positive stress. It works by allowing the root tip to come into contact with air. During this process the root tip die dies back through dehydration. Although this process is fairly stressful to the root system, it actually enhances it. Once the tip dies it promotes secondary root branching along the length of the root. Once these secondary root tips come into contact with air, they too become air pruned which stimulates more root growth. Much like pinching out the top of the plant to create a bushier plant, allowing the root tip to dry creates a more branched root system within the growing media. Air pruning can be done successfully with rockwool blocks by placing them on a wire rack allowing air to pass underneath them, or with specialized air pruning pots such as ‘Smart Pots’ or ‘Air-Pots’.
Plants that are grown for their aromatic qualities can sometimes benefit from brief periods of humidity stress during their final stages of development. As the leaves and flowers reach maturity, a sustained drop in RH can cause the plant to production more essential oils. Apparently this is a defense mechanism to further protect their leaves and flowers from the dry air.
The following are just “reports” from individual growers – so please take with a pinch of salt!
Give Me Thrips!
One grower we spoke to is now getting consistently better results now that he controls a small population of thrips in his indoor garden! This grower had a dialed in semi-closed garden and was used to getting the same yield time and time again. After a short two week holiday he came back to discover thrips had invaded and many leaves were damaged. He was not surprised when it came to harvest time to find the crop yield was significantly down. However, on the next grow he introduced the predatory insect amblysieus cucumeris in controlled release sachets. He found that by introducing these sachets every four weeks it kept the thrips numbers down to a minimum without having to regularly spray. To his surprise he found the next crop yield was up on his pre invasion average. The next crop he continued with the controlled release sachets to keep the thrips under control and again found his yield to be better than before. Is it possible that the very small amount of thrips feeding from his plants were mildly stressing the plant in such a way that it improved results? Difficult to believe but this grower was convinced.
A grower had four different 4ft grow tents in one room. Each tent has one big plant in and is harvested every two weeks. That was the theory anyway… all was going well until one problem plant just wouldn’t grow. It was fine two weeks into vegetative growth and then just stopped and the leaves started to curl! At first he thought it was over fertilization, so he ran the nutrient strength much lower, one week passed and still nothing. He then thought it must be over watering so he left it to dry more between irrigations, another ten days passed and still nothing. He noticed some of the roots had started to look slight brown so he disinfected the nutrient solution with an ozone sterilizer. Still the plant looked alive, with green curled leaves but another week passed and it was still not growing! All his friends told him to stop wasting his time and rip it out and start again but he wanted to fix it. Another two weeks passed with various attempts to fix the problem being unsuccessful until he decided to give the plant a full strength dose of nutrient solution with a ppm of 1200 (2.4mS). Boom! The plant sprung back into life the next day and grew like crazy! This poor stressed plant had been in veg for nearly 8 weeks, when it usually took 2. The long and the short of it is that although this plant did nothing for 6 weeks other than look half dead, it turned around to yield the most he ever got of one plant. Could this be down to its poor stressed out vegetative cycle? Maybe…
Another grower took cutting in pots of coir. Rooting times were 12-14 days – at this point he could see roots appearing at the bottom of his pots. On one occasion, four days after taking the cuttings, he was inspecting them when his phone started to ring. He picked it up and got distracted. Ten minutes into the call he remembered that he’d left the propagator lid off. He rushed back to his cuttings and found that many of the larger leaves had started to wilt. He quickly sprayed the lid and put it back on to quickly raise the humidity. The cuttings came back round but he was sure the stress would make the cuttings weak so he took some more as a backup. On the eighth day he checked his stressed cutting and was surprised to find that a few had roots at the bottom already! On the tenth day they all had roots at the bottom. On the next batch he did them as normal and was back to the 12-14 day turnaround, so on the batch after he purposefully took the propagator lid off on the fourth day and waited until most had slightly wilted. Once again the cuttings had visible roots between days eight and ten, as few days earlier than before. He now swears by this technique…
This grower loves his mouth to burn, and a few years back grew chilies using an ‘Autopot’ system where a valve tops up a small reservoir of nutrient solution in the bottom of the pot. It was mid July and the plants were in full swing and producing loads of fruits. One particular variety was quite mild and great in salads. One day the grower shuts off the supply valve in order to fill the reservoir with fresh water and nutrients, after filling he forgot to open the valve! An easy mistake to make but he didn’t notice for the whole weekend. Come Monday morning and all the plant were badly wilted but not unrecoverable. He opened the valve and let the plant recover. In the mean time he picked all the ripe chillies and they tasted the same as usual. He though the small developing chilled might abort but they all seem to continue growing. A few weeks later when he picked the ripe chillies the mild ones had developed a much more fierce heat level. It was the chilies that were developing during the drought stress, could it be that the short period of drought increased the spice level of the chilies?? The next set of fruits that came through were back to usual spice level so this grower is convinced that drought can influence spicier fruits.
During winter, this grower liked to cold shock his roots! He grew in three gallon pots using an organic potting soil with organic liquid nutrients. During the last three weeks of the flowering cycle he hand watered the plants with water at 55°F (13°C) once a week. He says that these cold irrigations shock the roots and the plant slightly, he does not result in any increase or decrease in yield but it does lead to a definite increase in essential oil production. The mild stress seems to work as a quality and flavor enhancer.
One grower finds that by introducing a few periods of drought stress in the last six weeks of his flowering cycle he improves plant vigor and produce quality. His technique is to let his coco coir dry out just to the point where he can see the first signs of the fan leaves wilting. He does this once during week 3, 5 and 7 and finds that the plants come back fighting after each short drought.
There’s plenty of food for thought in this article. And perhaps that’s the best thing to do. THINK about it. Before we all get instantly carried away the notion that stress can be good please read and heed this final word of warning:
PURPOSEFULLY CAUSING PLANT STRESS IN AN ATTEMPT TO IMPROVE RESULTS WILL NOT MASK OTHER GROWING INADEQUACIES SUCH AS POOR ENVIRONMNETAL CONTROL OR NUTRITIONAL DISORDERS!
If you want to have a play with positive stress techniques you should do so in a controlled fashion where all aspects of your grow are completely dialed in. Only then will you know if what you are doing is positively or negatively affecting your plants. It is one thing to mildly stress a healthy plant, but to stress an already stressed plant could lead to disaster!