Fertilizing herbs without burning them begins with one surprisingly useful idea:
fertilizer is not a growth accelerator with an unlimited throttle.
Indoor herbs need nutrients, but adding more than they can use doesn't force them to grow faster.
In containers, excessive fertilizer can increase soluble salts around the roots and eventually damage them.
So the objective isn't maximum feeding.
It's appropriate feeding.
And that starts before you even open the fertilizer bottle.
Fertilizing herbs without burning means supplying nutrients without creating an unnecessarily concentrated buildup around the roots.
In practice, that means paying attention to:
It also means accepting that fertilizer cannot fix everything.
A basil plant sitting in soggy soil and weak light does not necessarily need a stronger lunch.
Before adding fertilizer, look at the potting mix you're using.
Some commercial container mixes already contain fertilizer or other nutrient sources.
That matters.
If you've just repotted an herb into a mix containing fertilizer and immediately begin adding another feeding product, you may be supplying considerably more than you intended.
Read the bag or product information.
Find out what you're starting with before deciding what needs to be added.
When an indoor herb looks unhappy, fertilizer is an easy thing to reach for.
Yellow leaf?
Fertilizer.
Slow growth?
Fertilizer.
Basil looking personally offended by Tuesday?
Perhaps fertilizer.
But poor growth can also result from:
Adding nutrients won't repair those problems.
Diagnose the growing conditions before assuming hunger.
Plants can only use nutrients as part of the larger process of growth.
That growth also depends on suitable light, water, temperature and functioning roots.
Doubling the fertilizer doesn't double the sunlight coming through your window.
Nor does it double the plant's ability to use those nutrients.
Excess fertilizer can instead increase the concentration of soluble salts in the root environment.
So resist the understandable gardening instinct that says:
“A little helped, therefore a lot will help spectacularly.”
Plants are annoyingly unwilling to follow that logic.
Different fertilizer products have different concentrations and instructions.
That's why a universal instruction such as “use one teaspoon per liter” doesn't belong here.
One product's appropriate amount could be completely wrong for another.
Follow the directions for the specific product you're using, including:
The fertilizer manufacturer knows the concentration inside its bottle.
Your basil does not need us improvising a second set of instructions.
With a liquid or soluble fertilizer, concentration matters.
If the product is intended to be diluted, measure it rather than estimating with an enthusiastic squeeze of the bottle.
More concentrated does not mean more effective.
It simply means you're delivering more fertilizer in the same amount of water.
Never exceed the label concentration because you want faster growth.
If you intentionally choose a gentler feeding approach within what the product instructions allow, consistency is more useful than periodically compensating with a much stronger dose.
Correct concentration doesn't mean the fertilizer should be applied every time you walk past the plant.
Frequency matters as well.
How often an indoor herb needs supplemental feeding can change with:
That's why we'll treat light feeding schedule for herbs as its own subject rather than squeezing a universal calendar into this page.
An indoor herb's roots occupy a limited volume of growing medium.
Fertilizer salts that enter that container don't have an unlimited soil environment in which to disperse.
Over time, excess soluble salts can accumulate, particularly when fertilizer is applied too heavily.
You may sometimes notice crusty deposits on the surface of the potting medium or around the container.
But appearance alone doesn't tell you exactly what caused them; minerals from water can contribute deposits too.
The useful lesson is simpler:
don't repeatedly add more fertilizer than the plant and container need.
Overfertilization can damage roots and contribute to symptoms such as leaf-tip or margin browning, wilting or poor growth.
But be careful with diagnosis.
A brown leaf tip does not automatically mean fertilizer burn.
Similar-looking symptoms can have multiple causes.
Instead, consider the history:
Plant symptoms plus the feeding history tell you more than one brown leaf viewed in isolation.
First:
stop adding more fertilizer while you assess the situation.
Check what product you used, how concentrated it was and how often you've applied it.
If the container has effective drainage and the plant is otherwise being managed appropriately, watering in a way that allows excess water to drain through the pot can help move soluble salts out of the growing medium.
Allow the excess water to escape completely.
Do not leave the pot standing in the runoff afterward.
Avoid repeatedly flooding an already waterlogged plant in an attempt to “flush” it.
If the potting medium is already persistently soggy, deal with that problem first using Fixing Soggy Potting Mix Fast.
Before feeding an unhappy herb, check the basics.
Light: Is the plant receiving enough useful light?
Water: Is the growing medium actually ready for another watering?
Drainage: Can excess water leave the pot?
Roots: Has the plant become badly crowded?
Potting mix: Is it staying appropriately moist and aerated?
Fertilizer should complement a functioning growing system.
It should not be asked to rescue one.
If your parsley is standing in saturated potting mix in a dark corner, fertilizer is not the motivational speech it needs.
Don't automatically fertilize simply because you've just repotted.
First check the new growing medium.
It may already contain fertilizer.
Repotting also disturbs the root environment, so there is no reason to turn the same afternoon into a feeding experiment unless the product and growing situation actually call for it.
Let the herb settle into its new container and take the nutrients already present in the medium into account.
For the complete repotting process, see Repotting Rootbound Herb Plants.
Before fertilizing an indoor herb, work through this:
1. Check the potting mix.
Does it already contain fertilizer?
2. Check the herb.
Is it actively growing and otherwise in reasonable condition?
3. Check the basics.
Make sure obvious light, watering, drainage or root problems aren't masquerading as a feeding problem.
4. Read the fertilizer label.
Know the recommended concentration and application method for that product.
5. Measure rather than guess.
Don't make the solution stronger because the herb looks hungry.
6. Apply according to the product directions and your feeding routine.
More frequent isn't automatically better.
7. Allow appropriate drainage.
Don't leave the pot sitting indefinitely in runoff.
8. Observe the plant over time.
Don't expect an instant response and don't add another dose simply because nothing dramatic happened by tomorrow.
9. Adjust the routine rather than escalating randomly.
Let plant growth, conditions and product guidance inform what happens next.
Successful fertilizing herbs without burning comes down to restraint and observation.
Find out what's already in the potting mix.
Use a fertilizer appropriate for the plants you're growing.
Follow its directions.
Measure it.
Don't assume stronger is better.
Don't assume more frequent is better.
And don't use fertilizer to compensate for weak light, poor drainage or unhappy roots.
If you're wondering how often to feed, continue with Light Feeding Schedule for Herbs once that page is available.
If you've recently changed containers, see Repotting Rootbound Herb Plants.
If you're creating your own growing medium, see DIY Soil Mix for Herbs.
For the complete container-care picture, return to Pots, Soil, and Feeding.
Your herbs need nutrients.
They don't need an all-you-can-eat buffet.