Does Cooking Water Help Your Plants? We Asked a Pro Gardener

Rather than tipping your leftover pasta or potato cooking water down the drain, you might have read that there are sustainability and nutrient benefits to pouring it over your plants instead.

We asked an expert when you can use cooking water in your garden and when it’s better to ditch that leftover liquid.

Can You Use Cooking Water for Your Garden?

Using cooled cooking water to hydrate plants is a handy eco-hack that can contribute to water conservation and offer a beneficial boost to your soil. However, it’s not a substitute for fertilizer, and depending on what’s been cooking in the water, it can sometimes do more harm than good.

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Pros of Cooking Water for Your Garden

For John Murgel, Horticulture Extension Specialist, this is less about directly adding nutrients to plants and more about boosting the soil microbiome.

“By feeding the microbes, you boost soil activity, which ultimately benefits plants,” Murgel says.

Murgel explains that, particularly with starchy pasta or potato water, you’re adding carbon (organic matter) to the soil. Carbon helps support healthy plant growth by improving soil texture, increasing the availability of nutrients, and promoting moisture retention.

“[Applying cooking water to plants is] not about vitamins or complex molecules, it’s about a quick boost of ready-to-use organic matter that boosts soil microbial activity,” Murgel says. This means it won’t provide the essential nutrients that a proper NPK fertilizer will.

Cons of Cooking Water for Your Garden

While using cool, plain pasta, rice, or vegetable cooking water is usually a safe option, it’s not the best choice when other ingredients are involved.

“You want to avoid salted water, which nixes just about any cooking water, frankly, especially the ‘famous’ pasta and potato water, especially in repeated use,” Murgel says.

 Excessive salt in soil impedes a plant’s ability to absorb nutrients and moisture–that’s why salt is sometimes used as a weed killer. Chloride toxicity can cause leaf burn, wilting, and the eventual death of plants. 

“Water that has cooked meat and contains grease has the potential both to smell bad and to attract vermin, so it’s not recommended either,” Murgle says.

Studies also suggest that excessive cooking oil in soils can cause soil compaction, kill beneficial earthworms, and negatively affect plant germination and overall growth.

Be aware that regular use of super-starchy cooking water could promote the development of unsightly mold.

While mushrooms in plant soil can be an indication of healthy soil, you might consider these fast-growing fungi an eyesore.

How to Reuse Cooking Water for Plants

After draining your unsalted pasta, potatoes, rice, or veggies, let the cooking water cool to room temperature before using it to water the soil at the base of your plants. 

It’s best to use the water as soon as it’s suitably cool. Leaving starchy water to sit too long can encourage the growth of harmful microorganisms. 

You could pop the water in the fridge if you want to keep it longer, but always allow it to return to room temperature before irrigating your plants. Excessively cold water can shock plant roots, hindering growth.

If any plants show signs of stress after starting to use cooking water, they could be sensitive to the contents. This could be a particular problem for potted plants with a restricted root zone.

How to Fix or Revive Your Plants After Using Cooking Water

If you have been using salty cooking water without realizing the damage it can cause, it might be possible to revitalize your plants, provided the damage is not too severe.

“If salt builds up in the soil, it has to be flushed out with plenty of clean water,” Murgel says. 

Improving your soil’s drainage and watering infrequently but deeply can help dissolve salts and push any lingering crystals further down, away from your plant’s roots. These actions will also help when you find mold persisting on the soil’s surface caused by an excess of starch

FAQ

• You could use unsalted cooking water any time your plants need moisture. However, if the cooking water is particularly high in starch it could encourage the growth of mold and mushrooms. While these can sometimes be a sign of healthy soil, they can be unsightly. If you experience this issue you could try using it every second watering is an alternative.

• You may want to dilute the cooking water with tap water if you find that an excess of starch is leading to an increase in mold growing on the soil surface.