Control of Powdery Mildew Organically

At Glyn Bach, we hold a National Collection of Monarda; these plants are notoriously prone to powdery mildew, so we have had to learn how to control it, in order to keep our collection healthy and happy.

Mildew is around us, every day.  It is in the atmosphere and there are many different types.  Monarda only succumb to mildew when they are stressed.  To alleviate stress we have learnt the following:-

  1.  Plants need to have the right nutrients.  Monarda are heavy feeders and will not thrive if their feeding requirements are not met. Plants in pots soon exhaust the nutrients in the potting compost which will weaken them and they will succumb more easily to mildew.  We replace potting compost in pots annually and add Osmocote as a slow release fertiliser.  We also feed pots, from May onwards to the end of September, with a mixture of extract of seaweed and our home-made nettle tea.  Nettle tea is particularly advantageous as it encourages the leaves to produce a thicker coating. 
  • Whitefly and greenfly can weaken plants and spread disease so they must be controlled.
  • Plants in pots and in flower beds must not be allowed to dry out.  All Monarda require a certain amount of moisture and dry conditions will cause them stress which will exacerbate mildew.
  • Mildew descends on the plants in the mornings so overhead spraying during dry weather is advantageous as it washes off any mildew that is about to settle.  Be sure to spray before midday.
  • Plants should be routinely sprayed with an anti-fungal, from March onwards; as soon as growth begins to show, we spray weekly with our own organic mix (see mixture details below).  In April, nettle tea is added to this spray.
  • Humidity definitely increases the risk of mildew occurring but it is impossible to control outside so ensuring plants are healthy, making them more resistant to mildew, is important.
  • Many report that allowing air to circulate around plants is important.  We have not really found this factor to be too much of a problem.

Nettle Tea

We find nettle tea to be a brilliant free resource.  As soon as the nettles begin to appear in April, we pick the young shoots and place them in a 2 gallon glass container.  This is covered with water and left to mature for about 3 weeks.  As soon as the mixture gives off a pale green froth it is ready to use.  Every time we use some of the mix, we top it up with fresh nettles and water so we have a continuous supply until late Autumn.

Nettles contain formic acid which kills whitefly and greenfly. It is also rich in the following vitamins and minerals:-

vitamin A, various B vitamins (including B-1, B-2, B-3, and B-5), vitamin C, amino acids, calcium, fatty acids, folic acid, iron, magnesium, manganese, phosphorus, and potassium.

Nettle Tea makes a good all round food source for plants and is a great organic insecticide.

Mildew Spray

We make our own spray as follows:-

1 litre of water

½ tsp Bicarbonate of Soda

½ tsp Wood oil

½tsp Washing up liquid

Bicarbonate of Soda (Sodium hydrogen carbonate –  NaHCO3) is one of the best organic controls to use in the garden – it causes minimal damage to all wildlife and no detrimental impact has been noted on honey bees.   It creates an alkaline environment so powdery mildew cannot colonize the surface of the leaf since they need a neutral pH (around 7.0) to mildly acid to survive and thrive.

Washing up liquid is pH8 so also helps to neutralise mildew, but also acts as a surfactant for whitefly and greenfly. We use Ecover, but Fairy App

Wood oil, such as Murphy’s wood oil (classed as organic) is also a surfactant and it acts as a wetting agent on the surface of the leaf, allowing the Bicarbonate of Soda to neutralise the acidic mildew. 

The above products have limited impact on the environment and minimal to no impact on the important pollinators that we encourage into the garden.  We find this simple, cost effective regime is effective in controlling powdery mildew on our Monarda. 

Whilst we are primarily concerned with keeping our Monarda collection in pristine condition, we do also use our spray on our Roses and courgettes. We have also heard that milk neutralises mildew and black spot too, although we have never tried this.

The use of peat and peat free products for propagation purposes

At Glyn Bach Gardens we try to garden as organically as possible and use no herbicides or pesticides on our land.  When propagating our National Collection of Monarda we needed to know the best medium for growing our divisions (stolons from basal growth) in with minimal impact on the environment.  Our research into the three basic types of commercial compost uncovered the following facts:-

  1.  Peat compost.  Peat, from Sphagnum Moss, is singly the most organic medium but, it takes thousands of years to make and provides essential habitat for key species.  Harvesting peat releases methane and carbon dioxide is released into the atmosphere.  However, UK gardeners use only about 5 – 10% of the Irish peat harvested; the remainder is burnt as a fuel in stoves and on open fires in Ireland.
  2. Peat reduced compost.  Usually contain about 50% – 70 % peat and with the remainder made up of non-peat products.   See below for the breakdown of these products.
  3. Peat free compost.  This contains no peat at all.

Peat Free Products can contain any of the following:-

  1. Green waste from council tips.  Predominately grass cuttings in summer and woody material in winter.  However, people can put chemically treated grass cuttings (weed killers) and contaminants such glass, plastics, sharps and stones in the container.  Green waste is chloride and potassium rich which can lock up nitrogen and potassium; essential for plant grown. 
  2. Wood chippings from council tips.  Chipboard, MDF, wood, cardboard and paper can be shredded to make wood fibre but these can contain formaldehyde which is a known carcinogenic.  These are usually the main constituents of peat free compost.
  3. Food waste.
  4. Animal waste.  There are two parts to this one. (a).  Faecal matter from animals and humans (b). Slaughterhouse waste.  Both can contain harmful bacteria.
  5. Coir.  The fibre, found within the mesocarp of coconuts (the thick spongy layer within the fruit wall).  It makes a great filler for peat free composts as it retains moisture.  The snag is that this is imported from India and Sri Lanka by boat.
  6. Perlite and Vermiculite.  These are used to bulk out the compost, but vermiculite has been found to contain asbestos.

Methods to produce a good potting medium vary and are not yet standardised. Heat treatment may not be sufficient to destroy pesticides and pathogens.

The more expensive peat free products contain coir, animal waste and added nutrients; they omit green waste.  Cheaper products are heavily reliant on green wastes – these can be identified on the packaging by the fact that they are unable to germinate seeds.  Unfortunately there is currently no legal requirement to specify the actual constituents on the bags of peat free compost. Gloves should always be worn when handling any of these products and always wash hands well after use.

Trial of Composts

We decided to exclude all peat free composts, with the exception of Sylva from our trials as we did not wish to contaminate our land or soils in any way.  Until better methods of producing non peat composts is found, we will not use them and will look at our own alternative methods.

We carried out a controlled trial to see how well our Monarda divisions, for propagation purposes, developed effective root systems and how they continued to grow in the chosen medium.

We chose the following mediums to grow our divisions in:-

  • Home- made compost (our own non treated grass cuttings and fresh woody material.
  • Sylva Peat free compost
  • Peat based compost.

March 2019 Week 2

Small divisions were taken from assorted Monarda parent plants and placed in 9cm pots with compost as above.  Each compost also included the same amount of Osmocote slow release fertiliser. 

These were then divided in trays according to the type of compost used and labelled accordingly.

All pots were given equal levels of light, water and temperature.


March 2019 Week 3. After one week there was little notable difference in any of the cultivars.

April 2019 Week 1. After 3 weeks, a small number of divisions in the peat based compost and the home made garden compost had died. (Approximately 15% overall).

The Sylva peat free compost showed more promising results at this stage.  Nearly all the divisions took with less than 3% of losses.  One possible reason may be that the compost was more free draining and kept the roots much drier thus preventing the divisions from rotting off.

April 2019 Week 4.   By now the remaining divisions had rooted well and with the additional warmth and longer daylight hours, were beginning to put on good growth. 

The home-made compost showed the best growth spurts, the peat based compost in second place and the peat free compost showed the slowest growth.

May Week 2.  Divisions were now of a good size and ready to be potted on to 1 litre pots.  The home- made compost showed the best results with plants of a good size, with good root systems and strong healthy growth. However there were weed seeds in with the plants.

The peat compost also showed good results with plants of a good size, with good root systems and strong healthy growth, but slightly behind the home made compost.

The plants grown in the peat free compost were healthy, well formed, with no mildew, but were substantially stunted in growth, achieving 50% of the growth of the other plants.  (They looked like bonsai plants)


The peat free compost is the best medium for striking divisions as it is free draining so there is no damping off or rotting of the division material. But plants need to be potted up after three weeks into a better medium to allow for growth.

The peat compost retained too much water at the initial stage of striking the divisions so there were some losses.  However, those that did take grew well.

The home-made compost initially had some loss at the striking phase, but the plant growth and health more than made up for the losses. Pots did need weeding.

We reached the conclusion that we should make a lot more of our own compost as the healthiest and most organic of all the products tested, but supplement with either peat or Sylva to bulk out the compost.  Sylva is a great compost for striking cuttings and we would be happy to use that in the future.

Until manufacturers come up with a better way of producing peat free composts and list the ingredients used on the bags, we will avoid them out of necessity, to protect our environment and our Monarda collection.