What's the Weakly Weekly Method for feeding plants?

You hear the Weakly Weekly method mentioned by plant YouTubers and in plant care guides all the time, yet no-one seems to explain it. Maybe because it's so simple once you know what this method of feeding plants is and why it works.

To follow this method you just need a good quality fertiliser plus the water of your choice. I use tap water for most of my plants but do try to do use fish tank water when I get my timing right with a water change for my fish.

However don't confuse fertilisers and growth boosters. Most fertilisers provide essential nutrients at the level plants need. However growth boosters and tonics - like seaweed, Groconut or HB-101 - are not fertilisers.

Using a growth booster instead of a complete fertiliser can mean you end up with deficiencies pretty quickly, as they boost growth without replacing the essential minerals your plant used up to fuel that growth. Think of growth boosters like putting your foot down on the accelerator, and fertiliser like the fuel in your car.

What does 'weakly weekly' mean?

First-up, the 'weekly' part does NOT literally mean once a week. Put together, Weakly Weekly simply means to feed less, every time you water. 

The reason to follow this method is because it both reduces the risk of fertiliser burn by reducing how much you use, and reduces the risk of nutrient deficiencies by increasing how often you feed.

That's why you have to do both. Using a fertiliser at full-strength every time you water can otherwise risk fertiliser burn, even if it's formulated safe for indoor plants. On the other hand, using a fertiliser at a reduced dose, but not often enough, can result in the exact mineral deficiencies you're trying to avoid by fertilising in the first place!

The benefits of 'little and often' when feeding plants

It's important to know that not all minerals are mobile. If you have a deficiency in a mobile nutrient, like nitrogen, your plant can take that from its older leaves, so that new leaves can stay healthy, and so that growth can continue.

That's a common reason older leaves go yellow and die. That yellowing is your plant taking missing mobile nutrients - like nitrogen - from the leaves, when there isn't enough available for the roots to fuel the plant. That's also why you shouldn't remove yellowing leaves without fixing the cause first, as your plant's using those older leaves as life support to keep new leaves alive.

However not all nutrients are mobile. Calcium is a perfect example of an essential yet immobile nutrient. If there's a deficiency during growth, your plant can't just grab some calcium from old leaves to make up for what it's lacking.

A deficiency in an immobile nutrient like calcium can't be fixed. Instead you might see stunted or slow growth, deformed new growth, browning and early death of new leaves, buds that never flower, and split or cracked leaves. The good news is you can fix deficiencies in immobile nutrients for new growth.

How much fertiliser do you use for the Weakly Weekly Method?

Most fertilisers will tell you to feed 'every second water' or 'once a month'. However some are already formulated for the Weakly Weekly Method, and will give you a dose on the label for 'every time you water'.

That's why how much you change the dose depends on what the label says. If it recommends every second water, halve the dose. If the label says once a month, quarter the dose.

Some fertilisers have such tiny doses, that it's easier to change the amount of water, instead of the dose. Just don't do both. Check the examples below which will help this all make sense...

The Weakly Weekly method applied (with examples)

Label says: 6mls per 1 litre water, every second water. 

Weakly Weekly Method:

1/2 the dose: 3mls per 1 litre water, every time you water.
or 2 x the water: 6mls per 2 litres water, every time you water.

Label says: 1ml per 1.5 litres water, every second water. 

Weakly Weekly Method: 

1/2 the dose: 1/2 ml per 1.5 litres water, every time you water.
or 2 x the water: 1ml per 3 litres water, every time you water.

Why: Both labels give you the rate for every second water, so to convert that you can either feed half the amount (5mls becomes 2.5mls), or use double the water (1 litre becomes 2 litres).

Label says: 10mls per 1 litre water, once a month.

Weakly Weekly Method: 

1/4 the dose: 2.5 mls per 1 litre water, every time you water.
or 4 x the water: 10mls per 4 litres water, every time you water.

Why: The label gives you the rate for once a month, so to convert that you can either feed a quarter of the amount (10mls becomes 2.5mls) or use 4 x the water (1 litre becomes 4 litres).

Label says: 5mls per 1 litre water, every time you water.

Weakly Weekly Method: 

No change to dose: 5 mls per 1 litre water, every time you water.
No change to water: 5mls per 1 litre water, every time you water.

Why: This fertiliser is already formulated for the Weakly Weekly method and gives you the dose based on every time you water, so nothing needs to change.

So that's what the Weakly Weekly Method is, why it works, and how to work it out no matter what fertiliser you use. And yes, it does reduce the risk of fertiliser burn. However there are other tips you can use to help protect plants while giving them what they need.

Pro tip #1. Top watering's best

This method is not the only way to reduce the risk of fertiliser burn. Another pro tip whatever fertiliser or method you use, is to always top water. You need to water heavily enough that water freely pours out the drainage holes.

This flushes out excess minerals the plant hasn't used since you last fertilised, helping prevent mineral salts building up, burning roots and changing the soil pH. This also helps force out old, stale air, replacing it with fresh air. 

If you prefer to bottom water, replace at least one water every month or two with a good thorough top water to flush it out. Beware also of watering from a saucer or bottom reservoir all the time. Excess salts that do get flushed out, accumulate in the saucer underneath. That's because water evaporates but mineral salts don't.

So plants just end up absorbing higher and higher levels of salts every time you top-up the water in the saucer or reservoir. Best to fully tip out and replace the water if you prefer to water that way.

Pro tip #2. Pick a safer fertiliser

You can use the Weakly Weekly method with any fertiliser suitable for the plant you're watering, however another way to further reduce the risk of fertiliser burn is to look for a fertiliser with a lower salt index (SI), by avoiding certain higher-salt ingredients.

Two common fertiliser ingredients with a higher salt index are urea and chlorides. Urea is a popular source of nitrogen. Potassium chloride, like the name suggests, is a common source of potassium. Both are common in outdoor fertilisers. 

Urea is around 75% and potassium chloride is around 120% on the salt index. Some methods of measuring SI put both even higher. Although there's been a shift over time to urea-free and chloride-free agriculture, most outdoor fertilisers for the home garden - and many indoor plant fertilisers - still use both urea and chlorides.

Outdoors, urea and chlorides are easily washed down and away from roots, however indoors for plants contained in pots, delicate roots can't escape. Two urea-free fertilisers I know of are Dyna-Gro and Growth Technology (GT). One I know of that's free of both urea and chlorides is Growth Technology.

If you check the labels of both those brands, you'll see the directions for use are already formulated for 'every time you water' so they are made for the Weakly Weekly method, no change in dose needed. You may still like to halve the dose for winter though. But don't stop feeding altogether in winter. Here's why you should fertiliser in winter >