A Step-By-Step Guide to Correcting Things When Your Plants Aren’t Well


This section is written to help you tackle any problems that you may encounter during your hydroponic grows. These are just some of the more common problems that can occur. I’m assuming you’re using a quality nutrient for the crop that you are growing.  I haven’t addressed hydroponic nutrient deficiencies here as you can find more information on this here. Keep in mind, however, that nutrient deficiencies are extremely rare if you are using a reputable brand of nutrient. Anyway, let’s get into it! 

Rust-Like Spots on Leaves

There are several things that could be going on here.


  1. Fungus – Give the plants a treatment with a fungicide.
  2. Sap Suckers (Bugs) – Spidermite, Thrips, Aphids etc could be the cause of the problem. Look closely at the tops and the undersides of leaves for signs of bugs. Sap suckers remove the sugars from the leaves. Where this occurs areas of leaf tissue die. This can appear as rust-like spots on the tops of the leaves.
  3. Necrosis – Nutrient may be too strong. Necrosis generally appears during flowering. Check your EC and check that your EC meter is within range (use a conductivity buffer).
  4. Deficiency –  Have a look at the root system of the plants. If they are brownish in colour this is likely to be the problem. Other symptoms such as yellowing of leaves could also be apparent. Incorrect nutrient pH can also cause deficiencies in plants. Check that your pH is correct, and check that your meter is working correctly.  Some deficiencies can cause what appears to be rust-like spots on leaves. Perhaps think about switching to another nutrient if there is a question mark over the quality and/or the age of the product. Other nutrient problems can be caused by not mixing equals parts of 2 part packs or accidentally adding just part A or part B of a nutrient.
  5. Phytotoxity (poisoning) – The plant may be reacting to something that you are feeding it. Rust-like aberrations can appear on leaves due to this. Other signs of phytotoxity are sick looking plants, yellowing, and wilting. Dump the nutrient. Go to 1/2 strength nutrient. Don’t use additives – something that you are adding could be the problem.


Often it is very hard to analyse what exactly is the problem (several things could be  going on). For this reason a holistic approach to the situation is desirable.
Where Root Disease is Present:

  • Dump the existing nutrient and make up a fresh batch of nutrient at 1/2 to 3/4 the recommended strength at which you would normally run your plants. Add a fungicide to the water/nutrient such as Fongarid. Other products are also suitable for tackling root disease – check with your supplier. Depending on the stage of the crop some products may be more suitable than others. Check that your water temperature is maintained at 21 – 23 degrees C.  After a week dump the nutrient and this time don’t add the fongarid. Wait 14 days before adding preventives such as friendly bacteria.  Each time you dump the tank add friendly bacteria.


Where Root Disease isn’t Present


  • Double check that your EC and pH meters are working correctly.
  • Dump the nutrient in the tank and mix up a fresh batch of nutrient at 1/2 to 3/4 strength. Don’t use any additives other than the nutrient itself. Additives, particularly used in the very early stages of growth, can cause stress to the plant if used incorrectly or at too high dosages. If there is a question mark over the age or the quality of the nutrient that you are using you might want to replace it. Check the bottom of the bottles for signs of crystals and/or sediment. Before you add the new nutrient flush the plant medium with pH adjusted water.
  • Check for pests. Signs of pests are (often) tiny spots on the surface and underside of leaves, holes in leaves, web-like fiber on the underside of leaves, or webs on the plants
  • Spray the plants with a fungicide. Fungus could be the cause of the problem. 

Leaves start Yellowing and Dying (From bottom of the Plant Upwards – Particularly in early Flower)

This is usually caused by root disease. What’s basically happening is that the plant is suffering from major deficiencies. Firstly, check the root systems of the plants. If they are brown this will be a result of either Pythium, Phytophthora, or oxygen starvation (oxygen starvation is the most likely cause). Again, a holistic approach is necessary.



  • Check that your water temperature is lower than 25 degrees C (recommended 20-23 degrees C).
  • If you aren’t aerating the nutrient, then do so in the future.
  • Dump the nutrient tank. Refill and add 1/2 strength nutrient to what you’d normally be using at this stage of growth/flower. Add a systemic fungicide (that combats damping off fungi) such as Fongarid (check with your supplier for product information). Use for one week before dumping tank again. This time don’t add the fungicide. Wait one week then dump again. This time add root disease preventives such as friendly bacteria. Use as per instructions. It is highly recommended that you continue treatment with friendly bacteria in the future.  

Please note: Root disease is primarily caused by oxygen starvation. If you are using a tap water supply that is treated with chlorine it is improbable that you have pythium in the water supply. Pythium may come in via soil or dam, rain and bore water. 

Tiny White Spots on leaves

There are a couple of possibilities here. Firstly, you may have a fungal/mildew problem. A spray with a fungicide will take care of this.
Secondly, you may have an aphid problem. Aphids breed very quickly given the right environment. They are tiny, so they often appear as a mass of white spots. It is handy to have some form of magnifying device to have a closer look at the spots. If indeed, they are aphids, a treatment with a sucking insect spray such as Confidor will take care of the problem. If the plants are in flower, an organic spray may be more suitable.

Plants getting Too Big

Generally this is caused by the plants being switched down (18 hours to 12/12) too late. Another reason is that the genetics of the plant may not be ideal for an indoor situation. This isn’t likely, however, if you know the origins of the plant and whether it is suitable for indoor growing. For instance,lLong. lengthy equatorial genetics are less suited to indoor envrionments than short height, early flowering varieties. Know your genetics and turn them down to 12/12 at the appropriate times. Additionally, SCROG growing (netting plants down) is a very effective way to control plant height and create an even canopy level.  C pic below…

Extreme stress to the plant can, in some cases, cause unusual things to happen. Overactive gibberellins may possibly occur due to excessive heat (this is particularly true when night time temps are excessive).


SCROG Illustration

SCROG Growing Method

Tip Burning on Leaves


  • You may be mixing your nutrient too strong.
  • Or you may have a salts buildup around the root system.


Flush with pH adjusted water or a very weak nutrient solution and cut back on the levels of hydroponic nutrients that you are using.


Another factor that will affect salt levels is the size of your nutrient tank. What happens is that in recycling systems the plants take up water and salts at different rates. The plants will take up far more water than salts. This means that the water volume becomes less and the salts volume becomes more during the course of the day. As a general rule you should have a nutrient tank large enough to ensure its nutrient/water volume decreases by only 10 – 20%.
Also check that your salts meter is working properly. i.e. calibrate it. If in doubt check it against another meter that you know is working. (Or) ask your local supplier to check it for you.

More Severe Burning

This could be due to too small a nutrient tank. Remember, you want the volume of your nutrient tank to decrease no more than 20%.

This can also be caused by slack maintenance practices. You possibly need to be keeping a closer eye on your nutrient tank. This may mean ensuring that you top up the tank and adjust the pH daily. Particularly where recycling systems are concerned, EC and pH can be pretty unstable due to plant uptake of water and salts. The plants will generally take up a lot more water than salts. This can mean that the nutrient can become very strong. As a result of this the plants can burn due to high salt levels.

Leaf Curl Over

Leaves curling upwards can be caused by incorrect pH levels. It is generally a sign of nutrient deficiency (particularly Calcium). Incorrect pH can play havoc with nutrient uptake and cause deficiencies. Check the pH of the nutrient and that your meter is calibrated and working correctly. Also check your EC and your EC meter to ensure that you are giving your plants enough food.

Leaf Curl Under

This can be caused by over fertilization. However, again, this could be related to a deficiency. Check that the EC and the pH are within range. Ensure that your meters are working correctly and that they are calibrated.

Purpling of Leaves towards end of Flowering

Leaf purpling is fairly common towards the end of flowering. This is generally due to a Phosphorous deficiency. Towards the end of the plants life it can have problems up-taking Phosphorous (and Potassium among other things). Increasing the pH to 6.1 – 6.2 can help somewhat with this. Using additional PK products can also help. Try taking the pH to 6.2 and using a PK product (such as PK 13-14 or Heavyweight etc).


Another factor that can cause leaf and stem purpling is overly cold night temps (thermoperiod differential) and/or overly cold night and day temps. So the first thing to check is what your temperatures are during night and day.


Other than this, some genetics naturally turn purple or blue towards the end of flower.

Flower Rot

Flower rot is typically caused by Botrytis (also known as Grey Mould). Botrytis is caused by fungal pathogens. Its control really comes down to preventive management, as once Botrytis has established itself in your crop it is almost impossible to control. If you identify Botrytis in your crop it is likely to be in late flower; this makes it extremely hard to treat as it is undesirable to have traces of treatment agents left residually in your crop. The general rule tends to be to pull the crop right away, as Botrytis can spread very quickly.

In future, prevention is the key. Ensuring that you have adequate airflow around the plants will minimise the risk of Botrytis. Additionally, the use of a silica product greatly reduces the incidence of Botrytis (prevention not cure). Read more about silicon use in hydroponics here

Leaves wilting

Leaf wilt often signifies excess heat. Particularly while the plants are young and fragile, excess heat can cause the leaves to wilt. Put a thermometer at plant height, under the lights to check the temperature around the plants. If you don’t have a min/max thermometer, check the temperature 2 hours after the lights come on and every three hours thereafter until the lights turn off.

Plants Yellowing and generally unwell

Check your light timers. Lights staying on can cause all kinds of dramas. If your light timers are OK, check the root systems of the plants. If they are brown this will be the problem.

Flowers getting Offshoots

Light getting to the plants during their sleep period is often the cause of this. Are you staying out of the room during lights out? Are the light timers working correctly? Are the plants getting 12 hrs of uninterrupted sleep?
Another cause for this can be excess heat around the plants. Check the air temperature.

Room Temperature is too High

High temperatures can cause all types of problems to the plants. For instance, your medium and/or your nutrient could be getting too warm which can lead to oxygen starvation in the plants’ root zones. Also, high ambient air temperatures greatly affect plant growth. The plants can become stressed and susceptible to disease.

Try increasing airflow by adding extra air-in and exhaust fans. You can also purchase cooling devices for lamps such as water jackets and/or air-cooled shades.

The addition of an air cooler/humidifier in the environment can also help. What needs to be considered here is that humidified air conditioners increase relative humidity levels, which reduces transpiration, which reduces leaf cooling. The key is to use a humidifier and at the same time, increase airflow (via more exhaust etc).

A better option is to use a refrigerated air conditioner. However, I recognise that this can be an expensive option. Therefore, a humidifier/cooler may be a more viable option for you.

Where water is too warm (above 25 degrees C): If you can move the reservoir outside the room (only if it is cooler outside the room) do so.

You can also freeze bottles of water in plastic drink containers and throw these into the reservoir to bring down the nutrient temperature.  Devices are available for water-cooling but they are fairly expensive and energy hungry.

Plants Stretching (large gaps between internodes)

The most common reasons for this are:


  1. Genetics – Some strains, no matter what you do, will have large gaps between the internodes.
  2. Too hot during plants’ nighttime – Stretching can occur because it is too hot during the plants’ nighttime. This is related to the hormonal activity within the plant.
  3. Additives – The use of certain additives at the wrong times can cause problems. Stem elongation is largely due to gibberellin activity in the plant. The use of the wrong additives, at the wrong time can increase gibberellin activity.
  4. Low Light Levels – Your plants could be receiving too little in the way of light. If your plants are directly under 400 watt or more powerful HID lamps this shouldn’t be the problem. However, if your grow (18 hours) is too long under HPS lamps (red spectrum) then some stretch is bound to occur prior to going into the flower phase (first 1 to 2 weeks of 12/12). This should remedy itself in the flower phase.
  5. A remedy to large gaps between the internodes can be the use of an anti-gibberellin product. Check with your supplier for details. Be aware, however, that there are potential toxicity problems associated to the use of some of these products.
  6.  Low EC/Not Enough Nutrient – A plant that isn’t receiving enough nutrition will stretch. Check that your EC meter is working correctly. Check that your EC levels (eg. 1.8 in grow, 2.8 in bloom etc) are correct.

Smaller Yields than Normal

This could be a result of many different factors. These will likely be primarily environmental.  Read more about hydroponic growing fundamentals here. And Maximum Yield Hydroponic Growing here.

One factor that can affect your final yield is the age of the lamps that you are using. If, after saying to yourself that you are doing everything to spec, and everything that you did the time before, think back to when you last replaced your lamps. Lamps will burn brightly for over 20,000 hours (in the case of HPS). However, while their lumen output may be reasonable, the colour spectrum drops out over time. Because of this, the red spectrum that stimulates hormonal changes in the plant may not be quite what it was when you first purchased the lamps. For this reason, it is advisable that you replace your lamps at least every 10 months. Many growers replace lamps every third crop.


‘C’ Growroom pests

Plants Looking Very Sick/ Plants Not Growing (or) Growing Very Slowly

As I’ve pointed out – plant health and vigour relies on a holistic approach to growing. If your plants are struggling, check and recheck all of the key essential elements (temp, humidity, airflow = carbon dioxide/ oxygen, light, and nutrition/uptake).

White Slime on Hoses etc in Nutrient Tank

This is caused by bacteria building up around the hoses etc in the nutrient tank. Sometimes this buildup can be very pronounced. This needn’t be a problem; it will depend on which forms of bacteria are present in your nutrient tank. If you are using friendly bacteria you needn’t worry. If you are using a sterilising agent such as hydrogen peroxide this bacteria shouldn’t be present. Read more about beneficial bacteria and sterilization here…



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