How Not to Kill Your Houseplants Part II: Fertilizing
In nature, plants aren’t confined within pots. (Duh.) They’re also not limited a pot’s worth of soil and soil nutrients.
Of course, in nature, your plant’s ecosystem would naturally restock the nutrients that your plant needs to survive by soaking up degrading local plant and animal matter or even rock dust. Fertilizer makes up for the lack of natural nutrient renewal to your plant’s soil, by adding nitrogen, phosphorus, and potassium (or N, P, and K, for short).
Fertilizing can be complicated, especially for anyone (ahem, me) who slept through chemistry. So here are a few useful tips.
What are those three numbers?!?!
When you pick up a bag of fertilizer, it almost always had three numbers separated by dashes, typically 16-16-16, 16-6-4, 3-20-20, etc. The three numbers are the percentages of N-P-K (nitrogen, phosphorus, and Potassium) in your fertilizer, and the leftover balance (out of 100) is typically water or other neutral biodegradable carrier. When in doubt, opt for a balanced mixture – something like 10-10-10 is a standard all-around fertilizer. This ensures that your plant gets everything it needs to grow and produce healthy roots, flowers, and/or fruits.
OK - so why would you buy anything other than a balanced fertilizer?
While most plants like balance, some plant types like succulents or citrus are a little more finicky. If you’re worried, just buy the pre-mixed fertilizer that’s made for your plant type.
OK, but let’s say you already know that and you’re just a serious plant warrior mamma who wants her plats to grow in a certain way. Mixing up fertilizer ratios can also give you precise control over how your plant grows.
If you’re looking for rapid, tall, lush growth, power up on the nitrogen. A good springtime, growth formula is 16-6-4, or something close. You’ll probably be able to find it pre-mixed, maybe even labelled “spring.” The only downside, nitrogen is a bit more acidic than the others – while this is great for citrus, some plants can’t handle it.
Want to “bulk up” with thicker roots or big, juicy fruits and flowers? Focus on phosphorus, and turn down the nitrogen a bit to direct your plant’s energy away from the tips of its branches – try 3-20-20. (Don’t use too much phosphorus on proteas, they’ll binge-eat it and die.)
Potassium helps your plants general welfare, promoting disease and insect resistance, protein production, and efficient water use. It also helps continue growth even if you’re minimizing your nitrogen.
How long does fertilizer last?
When you buy a plant, assume any fertilizer in the pot’s already been absorbed. Give your new plant a week to adjust to its new digs and then start your fertilizer routine.
Solid or “slow release” fertilizer lasts anywhere between 1 – 6 months and liquid fertilizer typically about 2 weeks, but with synthetic fertilizers functional duration can vary widely. Many slow release and stake fertilizers use a special coating that only allows a tiny bit of fertilizer to be released each time you water, extending the life of the fertilizer. Always check the package for lifespan and adjust for growth or dormancy season.
Is there such thing as too much?
It’s difficult - but not impossible - to over-fertilize house plants. Just like (most) people, a plant eats when it’s hungry and stops when it’s full. As you water, fertilizer that the plant hasn’t consumed is slowly drained out of the soil. As long as you let water drain after watering and re-pot and replace soil every two years, buildup most likely won’t be an issue.
That being said, if your leaves are turning brown and curled at the edges, it’s possible that too much fertilizer is burning the roots. Lay off for a few months, and when you re-introduce, use less or try a different combination.
If you notice white “lime” on the surface of your potting soil, it means that you’re a bit heavy on the nitrogen, and your soil’s acidic. Liming is the result of the plant rejecting the additional nitrogen and its residue being left in the soil. It won’t hurt the plant, but you can re-pot and adjust your fertilizer mix or frequency of use if you don’t like the look.
As a general rule, synthetic fertilizers are more likely to be over-used than organics, as they’re fast-acting. Don’t use more than a small pinch of the solid stuff, or a very diluted liquid (1 tbs of concentrate powder per gallon of water, or the pre-diluted stuff) heavily sprayed or lightly watered into the soil around the base of your plant.
Growing season vs dormancy
Plants go through natural growth and dormancy seasons, with growth typically happening from spring through late summer and dormancy in fall and winter for most varieties. Plants naturally need much less food and water when they’re dormant than when they’re actively growing.
But keep in mind – this guy lives inside! Do your radiators make your home warmer in winter? Or does the room you’re in get better light during winter months? Typical growing season patterns might not apply inside your house. Fertilize consistently for the first month and make note of when they seem to be growing the most actively.
Just like you should slow down your watering routine when you’re plants dormant, only fertilize every two months or so. (Less for slow release.)
Organic vs. synthetic
While typically more expensive, organic is almost always healthier for your plant and the environment. The benefit of synthetics are that they tend to work more quickly, and can be engineered to last much longer. Synthetics are also often sold in mixtures that contain pesticides.
Can I make my own?
Yes!! There are tons of ways you can make organic fertilizer at home, just know that homemade fertilizers won’t naturally be as strong or as balanced as the stuff you buy in the store. If you’re curious, you can look up N-P-K content of natural fertilizers like compost (1.5-.5-1 to 3.5-1-2), manure (3-2.5-1.5 to 6-4-3 or 1-1-1 depending on species), or other common home fertilizers like coffee grounds, used cooking water, fresh fish tank water or seaweed online. Grow network put together a great list of homemade fertilizer recipes and their relative N-P-K ratios.