Phoenix Gardening Course – Sow & Grow. Chapter 4: Pests & Diseases

Read on for the final online instalment of our Urban Gardening Course all about raising plants.  This chapter deals with common pests, looking at acceptable organic solutions, including crop rotation and companion planting.

It might seem that you are facing unfair odds against pests and diseases which will attack your plants but take heart!  If you support your soil and raise healthy plants, they will withstand minor attacks.  Don’t forget – we need the pests to entice the predators into our garden spaces – don’t reach for the spray at the first sign of trouble.


1. Habitat & Biodiversity

You can provide habitats to encourage biodiversity: Pond, hedge, grasses, nettles, flowers that attract pollinators and predators, piles of wood, stones.  Don’t be too tidy!  Leave a corner to go ‘wild’ and leave it untouched.  Mixture of plants = more beneficial creatures = less pests & disease without the need for chemicals.  Use our Organic Weed Control Quick Reference Pros & Cons to help with other ideas.

2.Identify your common pests

There are a wide range of beasties waiting to sink their proverbial teeth into our fleshy crops.  Know your foe! Below is a list of the most common, with some suggestions on how to dissuade them.

Slugs & snails:

  • Physical barriers
  • Slug pubs
  • Ferric phosphate: slug killer (NB: a word of warning on using this organically approved slug pellet – studies have shown the compound has a negative impact on earthworms)
  • Nematodes (biological control)

Birds, Rabbits, Rodents, foxes, badgers:

  • Physical barriers such as netting, fleece, cloches; prickly material can work for mammals

Insects: including aphids (greenfly, blackfly, whitefly, root aphid), fly (carrot, onion, cabbage), root ants, earwigs, wasps, red spider mite, caterpillars (cabbage white, pea & leek moth), eelworm, millipede, cutworm, wireworm

  • Physical: barriers, mulches, traps, pinch out stem tips
  • Hand picking
  • Spray with strong jet of H2O
  • Spray with soft soap
  • Netting
  • Nettle spray
  • Companion planting
  • Crop rotation, timely sowing, resistant varieties


These can harbour pests & disease and compete for water, light, nutrients, so in your planted beds you might want to keep these common weeds at bay.

Ephemeral weeds (set seed fast), such as Shepherds Purse, Annual meadow grass, Hairy Bittercress; Perennial weeds can smother crops Such as Ground Elder, Field Bindweed, Couch Grass


Plant diseases

3. How to tackle your foe organically

Other weapons in your armoury against pests.

3a. Acceptable Pesticides for organic gardening

  • Insecticidal soap (fatty acid)
  • Rapeseed oil
  • Pyrethrum (NB: this is fatal to bees, so use in the evening after bees have returned to nests)
  • Whitefly killer (plant extract)
  • Garlic oil
  • Winter tree wash: enzymes to dissolve insect eggs
  • Sulphur (fungicide)
  • Sodium or Potassium bicarbonate (fungicide)

3b. Recipes for homemade organic treatments (NB: use with caution, natural products can still be toxic; wear gloves when handling)

Mix one tablespoon each of bicarbonate of soda, vegetable oil & insecticidal soap together in one gallon of water. Shake mixture well and use as a fungicide spray

Garlic fungicide treatment: mix 10 cloves of garlic with one pint of water in a blender. Strain & spray.  Also good for repelling aphids and slugs & snails.

Milk is a safe method to treat powdery mildew, especially on cucumbers mix 50:50 milk & water.

2% insecticidal soap: mix together 5 tablespoons soap to 1 gallon of water, 1 tablespoon soap to 1 quart of water.

Cooking Oil: To help the solution stick, add 2 tablespoons of vegetable oil per gallon of water

Vinegar: To make a spray that also targets powdery mildew, add a teaspoon of cider vinegar per gallon of water

Garlic or Pepper: To help repel chewing insects, add a teaspoon of ground red pepper and/ or garlic per gallon of water

Nettle spray (brassicas and leeks) to deter caterpillars: Cut 1kg of nettles, put in an enamel bucket add 10 litres of water, leave 24 hours, strain and spray crop x3 in one day.  Left to mature for up to four weeks, can be used as an anti-fungal treatment and growth stimulant.

A more potent anti-fungal remedy and preventative is made by boiling 10g dried horsetail (the weed not the animal!) in 2 litres of water for 20 minutes. Add 8 litres water and stir for ten minutes; strain and spray soil around base of plants in the evening; this can be repeated on three consecutive evenings in bad cases, especially if it’s been very rainy.

3c. Physical pest control

  • Grease bands
  • Yellow sticky traps
  • Codling moth traps
  • Slug pubs
  • Copper tape
  • Cabbage collars
  • Fleece (horticultural)
  • Enviromesh
  • Netting

3d. Biological pest control

A list of common pests and their biological solution.  You can purchase these online or at garden centres.  They can be pricey, so decide if you really have an infestations, or if you can live with it!

  • Slugs & Snails: Nematode
  • Whitefly: Encarsia Formosa
  • Red spider mite: Phytoseiulus
  • Aphid: Adalia bipunctata
  • Vine weevil: Steinernema kraussei
  • Cabbage butterfly: Bacillus thuriensis

4. Companion Planting

By planting carefully, you can utilise plants’ interaction to help minimise plant disorders and attacks by pests.  Companion planting provides shelter, support and shade for growing.

  • Plant crops that require similar conditions together
  • Sturdy plants can support climbers – a classic is the ‘Three Sisters’: Sweetcorn, Beans and Squash planted together in mutual support
  • Tall plants give wind shelter to large leaves e.g. courgettes & corn
  • Use the shade provided by tall plants for crops needing less sun, e.g. lettuce
  • Avoid monoculture: Insects will be confused by landing on non-target plants
  • Mix plants to attract beneficial insects & confuse or repel pests
  • Friends: pollinators, predators & parasites; Foes: pests
  • Soil life, Nutrients, Channels for air & water

 4a. Plants to attract insect friends


  • Nettles
  • Yarrow
  • Fennel
  • Cosmos
  • Golden rod
  • Pot marigolds (calendula)


  • Buddleia
  • Lavender
  • Sedum
  • Comfrey
  • Marjoram
  • Fuchsia
  • Phacelia
  • Poached egg plant
  • Borage

5. Crop Rotation

Crop Rotation

By swapping different plant types to different beds every year or so, you can break the cycle of soil borne pests and diseases.  Don’t get too hung up on strict rotation plans, you only need to rotate if you have a problem.  Garden Organic has a good guide to what crop rotation is all about and what the traditional approach is if you’re interested in more detail.

  • Better soil conditions for following crops
  • Avoids soil nutrient depletion
  • Dying roots leave nutrients & channels in soil for improved air & water movement
  • Use green manures or mulches wherever you have an empty bed.  Use our at a glance table of Green Manures and check out our members trial.

Missed the previous installments?  Catch up here: Chapter 1: What, When, Where, How?  Chapter 2: Sowing seeds & potting on  Chapter 3: Supporting your soil

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