It is always smart to do some research before mixing chemicals, and it is also smart to work smarter not harder.
Some types of fungicides should not be applied at the same time as fertilizers. Many fertilizers require water to allow them to leech into the soil and nourish the plants. Fungicides on the other hand should be confined to the area where they are carefully applied and may be harmful if spread by watering.
If we are talking about a food garden, use extra caution and read all warning labels on your fungicide. Most fungicides have a safety limit for consumption.
If the fungicide is spread unevenly you may end up with food overburdened with the chemical and some that are not treated.
Fungicide is generally used to treat blight in food applications and you should reach out to local agricultural extension for instructions to contain this dangerous garden problem.
Lawns and gardens offer more flexibility for applying fungicide and fertilizer at the same time.
What To Consider When Mixing Fungicide With Fertilizer
The first thing to do when you are looking at mixing lawn and garden chemicals is to read the labels carefully.
Some labels are very helpful and may even let you know what they can be mixed with and some will not.
After reading the label it is important to perform a jar test.
How To Perform a Jar Test
Read the labels of your fertilizer and fungicide and plan out how much of each product you will need for a gallon of mixed product.
Divide the quantities down to a pint by dividing by 8 or divide by 4 for a quart. Get a jar that is the correct size and do a test mix.
You are looking for three reactions when you mix the chemicals
Physical Compatibility
Some chemicals will simply not mix. They will settle into layers like oil and water. A jar test can show you if your chemicals can physically mix.
Put on gloves, a respirator if necessary and eye protection before performing a jar test.
Be sure to start with at least half a jar of water before adding your chemicals.
Add your chemicals in this order:
Wettable powders
Liquids and flowables
Emulsifiable concentrates
Mix well between each addition and observe any changes after each addition.
Chemical Compatibility
Some chemicals will react with each other and form globs or particles that can clog or even destroy your sprayer.
Have you ever added vinegar to milk? You have seen the curdling effect when these two combine. Some lawn and garden chemicals can react the same way.
When you perform a jar test let the chemicals sit to be sure that no particles are formed as they will settle on top or bottom of the liquid.
If your mixture produces these granules do not apply them at the same time.
Chemical Changes
If you are mixing 2 chemicals in your jar test if you see fizzing, unexpected color changes, or heating or cooling of the mixture then you have a chemical change.
After two chemicals have reacted together they have changed their chemical makeup. You no longer know what you have or what it will do to your plants.
Do not apply chemicals that have reacted together as you do not know if they are effective anymore. They may even cause harm to your plants.
Activation
Another concern is how each chemical is activated. Some chemicals need water to activate, but some should be kept dry for a period of time after they are applied.
Be sure your chemicals are compatible in this way.
This video shows a very safe way to perform a jar test.
Pay attention to the safety information in this video.
Can I Apply Fungicide and Fertilizer At the Same Time?
If you have done a small jar test and the results were good and you did not notice any physical or chemical incompatibilities or chemical changes then you can apply fungicide and fertilizer at the same time.
If you are using a sprayer be sure to mix and use it right away and if you are doing a dry product mix in the spreader just before application.
Every lawn and every gardener has a different process and may give you different advice, but if you perform your own tests you can be confident in your unique blend.
This lawn professional recommends his favorites and shows his dry product application on a lawn that needs a boost.
He promises that he can turn a regular lawn into a golf course lawn! Sounds like he knows what he is doing.
Fungicide and Fertilizer Mixing Chart
While you should always perform a jar test when mixing lawn and garden chemicals, there are some popular fungicides that can be mixed in most circumstances.
| Fungicide | Fertilizer | Notes |
| Bonide Neem Oil | Silica | Neem Oil can be difficult to mix with water as it separates when not constantly agitated. |
| Garden Safe Fungicide | Not Formulated for Mixing | This is a pre-diluted product and is not suitable for adding to a sprayer and mixing. |
| Bonide Liquid Copper | Water Soluble Fine-Grained | Copper application can last many months, copper fertilizers are also available. |
Depending on your style of gardening and your chemical choice you should be careful to always check the labels even if every component is organic or natural.
Substances can react in unexpected ways and as these chemicals age, they may behave in different ways.
A jar test and careful research can save your plants and your sprayer.
Mixing Fungicide and Fertilizer
Many gardeners, landscapers, and caretakers want to minimize the passes they need to take over their lawn or garden.
Proper mixing of fungicides and fertilizers can make that possible. It is even possible that applying these together can be beneficial and make both works better.
Be sure to perform a jar test each time you do a new mixture or new ratio of chemicals into your sprayer to be sure your mixture is compatible and won’t clog your spraying equipment.
When considering adding fungicide to your garden be sure that you are also taking all the precautions to stop the spread of the fungus to other plants or neighbors yards.