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Ask A Plant Pro: Fertilizer

hand adding scoop of Frass fertilizer to pot of soil

Every month we answer a customer-submitted plant question, think of it like a Dear Abby column but for plant-related questions. This month's submission:

Dear Plant Pro,

How often should I fertilize my plants? 

- Nutrient deficient in Norwalk

Fertilizing is likely the most neglected aspect of plant care for a majority of houseplant parents, and usually for the simple reason that while they won't thrive without it, most plants will definitely survive without it. Beyond that, fertilizing can also be a confusing and overwhelming topic. Questions like 'What kind of fertilizer should I use?', 'How often should I fertilize?', and 'If I do it wrong, will I kill my plants?' come up often, so today, I'm going to break down fertilizer as simply as I can so you can start fertilizing your plants with confidence. 

Why do plants need fertilizer? 

If light is to plants as food is to humans, think of fertilizer as vitamins for plants. You can survive for a time without vitamins, but you won't be very healthy and eventually will start to have serious health problems. The same goes for plants. Plants need light to photosynthesize in order to survive, but if they're living off of light alone, they will start to struggle and without certain nutrients will eventually not even be able to photosynthesize properly. Plants in the wild obtain essential nutrients from the world around them - minerals in the soil and water, organic matter decaying around them, etc. Once we place plants in containers and bring them indoors, we take away their ability to obtain those nutrients naturally, so we have to supplement them with fertilizer. Most commercial potting mixes contain some amount of fertilizer, but often only enough for 3-4 months at the most, so I always recommend using fertilizer if it's been more than 3-4 months since a plant was repotted. 

What nutrients are most important for plants?

The big three that you'll usually see listed as a ratio on a package of fertilizer are:
  1. Nitrogen (N)
  2. Phosphorus (P)
  3. Potassium (K)
There are others that are also important, but not needed in quantities as large so for our purposes today we're going to focus on those 'big three'. Each supports different activity in plant development:
  • Nitrogen plays a big role in photosynthesis and tissue development, so it's especially important for leafy plants. 
  • Phosphorus is essential in cell division and helps transfer energy from one plant part to another. Plants grown for flowers often do best with fertilizer with a higher P number (higher phosphorus content). 
  • Potassium functions as an enzyme catalyst, which is important for plant growth. Potassium also regulates the opening and closing of a plant's 'pores' (technically called stomata), which is where that famous exchange of Co2, and O2 is happening, as well as where water vapor can escape plants. If the stomata aren't regulated properly, too much water can be lost through these pores, which leads plants to dry out too quickly. 

What do the numbers on the fertilizer packaging mean? 

Most fertilizers will have a ratio of the NPK content listed somewhere that looks something like 10-10-10 or 10-10-5. This ratio just indicates the relative content of the three nutrients, so fertilizer with 10-10-10 on the package means there is an equal amount of the three, while 10-10-5 indicates there is less potassium than nitrogen and phosphorus. 
Packaging will often indicate what type of plants a particular fertilizer is best for, but when in doubt a balanced ratio is usually a safe choice for any houseplant. 

What kind of fertilizer should I use?

Fertilizer comes in a myriad of options - from slow release stakes or pellets, to liquids, granular mixes, and even foams. Which kind you should use will depend on a number of factors particular to you.

different kinds of fertilizer

  • Slow-Release: People who are prone to forget or are too busy may prefer a slow-release type since there will be a period of time where you don't have to fertilize while the pellets or stakes are still actively releasing fertilizer. I find pellets feed plants more evenly, whereas the stakes can sometimes cause burn due or uneven growth to all the fertilizer being concentrated in a certain area of the pot. 
  • Liquids: Liquid fertilizer, which is often meant to be mixed into water, is a great option for folks who use a watering can to water their plants or bottom-water. Liquid fertilizers are easy to apply evenly, and as long as they're mixed at the correct strength come with low risks for fertilizer burn. Some liquid fertilizers can also be applied foliarly, where the mixture is sprayed on the leaves instead of applied to the soil .
  • Granular: Granular fertilizers are great for mixing into DIY potting mixes before potting, or for 'top-dressing' a potted plant, where you mix the fertilizer into the top layer of potting mix. A wide range of organic fertilizer (as opposed to synthetic) comes in granular form, so if that's priority for you, this may be the option you'll want. 
  • Foams: Fertilizer foams are a relatively new addition to the world of fertilizer and some people swear by them. This type comes in a foaming pump bottle, and you apply the foam to the surface of the soil, or mix it into water. The one time I tried this type of fertilizer, I found it a little awkward to use, and hard to tell if I was getting the dosage right, so I don't necessarily get the appeal and prefer other types of fertilizers, but I know many love the foam. 

fertilizer watering can

 When and how often should I fertilize? 

You'll often hear people talking about fertilizing during the 'growing season', which won't be the same for everyone, since where you live, the plants you have, etc will all be factors in how long plants are actively growing. For me, because southern California is sunny and warm most of the year, nearly all of my indoor plants put out new growth year-round. If you live in a climate with a more distinct winter and the accompanying shorter and colder days, you may notice your houseplants slow down or stop growing altogether in the winter unless you use supplemental technology like grow lights and heat mats. Because there are so many variables to a 'growing season', my rule of thumb is simply- if your plant is actively growing, you can continue to fertilize. 

When it comes to how often you should fertilize, there's a popular old adage in the gardening world- "feed weakly, weekly", meaning, if you apply your fertilizer at a weaker strength, you can use it more frequently without risking fertilizer 'burn' or other damage to your plants. This is especially good wisdom when first trying a new fertilizer, particularly if it's not a time-release type fertilizer. All fertilizer packages should indicate how to apply it, which usually involves either mixing it with water or mixing it into the soil, so to apply it at a weaker strength, just use half as much as the instructions indicate, with the same amount of water or soil as indicated. 

You'll also want to pay good attention to all the instructions on the package of whichever fertilizer you choose- some are formulated to be used less often than others (even if mixed at half strength), so be sure to pay attention to that information so you don't risk over-fertilizing your plant. 

Fertilizing can definitely be a daunting part of plant care, but I hope this overview has you feeling less overwhelmed and ready to add this important piece to your plant care plan! As always, if you have a plant-care question, fertilizer or otherwise, fill out our Plant Care Help Desk Questionnaire or ask a question on TikTok

We're also very excited to announce a new addition to our fertilizer lineup at Folia- Frass by Grubetts!

Plant potting set up with plant, potting soil, a planter, and bag of Frass fertilizer

Created as part of a closed-loop system, right here in southern California, frass is an up-cycled product of Black Solider Fly larvae- similar to worm castings. By diverting it from landfills and putting those flies to work, Frass by Grubetts transforms FDA approved, pre-consumer food waste (which accounts for 40% of all food waste in the US) into certified organic fertilizer that not only feeds your plants, but creates healthy soil too. Frass contains chitin and beneficial biology which are known to assist in nutrient cycling, aid in nutrient uptake, reduce nutrient leaching, stimulate natural immune responses in plants, prevent predator insects and pests from harming plants, and promote resistance against soil pests and parasitic nematodes.

Use this dry, granular fertilizer to prep your soil when potting, as a top-dressing to encourage growth, or make frass 'tea' with it to use as a root drench or foliar spray. Frass can also be used in outdoor gardening, and as a compost starter. The 3-2-2 NPK formula is perfect for growing lush foliage, and the non-water soluble nitrogen releases slowly over 3-4 weeks, which minimizes risk for fertilizer 'burn'. 

We've been evaluating fertilizers for years, hoping to find one we felt was a good addition to the liquid fertilizer from our friends at Good Dirt that we already carry and this was the first one that ticked all the boxes:

  • Created as part of a closed loop system that's addressing multiple environmental concerns
  • Organic matter based formula that doesn't contribute to environmental pollution
  • Sourced locally to reduce emissions used to transport it
  • Packaged using more sustainable materials
  • And best of all- helps build healthy soil
We think you'll love it as much as we do. Available in-store and online (at last, a fertilizer we can safely ship).