Fertilizer for Self-Watering Pots

Why a reservoir pot changes the fertilizer question

In an ordinary pot, fertilizer salts are managed by the watering itself. You water from the top until it runs out of the bottom, and the excess carries dissolved salts out with it. That is why watering thoroughly is the instruction every extension service gives, rather than watering little and often.

A sub-irrigated pot does the opposite, permanently and by design. Water enters at the bottom and moves up. Nothing runs out, because there is nothing to run out of. University of Maryland Extension states the consequence directly: fertilizer salts may accumulate on the surface of the potting media as a white crust, especially if bottom watered, and can cause stem collapse at the soil line.

University of Missouri Extension says the same thing from the other direction - salts may form a white accumulation on the soil surface if plants are watered regularly from the bottom, and occasional watering from the top helps wash out the salts. Both services are describing the exact mechanism a self-watering planter is built around.

How we judged these

Four criteria, in this order. They are deliberately different from the criteria you would use for a plant in an ordinary pot.

1. Does the analysis include calcium and magnesium? This is the criterion that reorders the whole category. In a conventional pot you supply calcium by working dolomite or lime into the surface. In a planted sub-irrigated pot the root ball sits on a wicking column you are not going to disturb, so a feed that brings calcium in the water is doing a job the alternative cannot. Only one liquid here publishes a calcium figure.

2. Is the dose published, and by whom? Over-application is the specific failure extension services warn about, and the dose is the only defense. We rank a published rate above an unpublished one every time, and a rate published by a university extension service above a rate published only by the maker.

3. How much salt does one feed leave behind? A weak analysis applied often leaves less residue per application than a strong one applied rarely. In a pot you cannot flush on demand, weak and frequent is the safer shape.

4. Can you dilute it? A liquid or a soluble powder can be taken down to a quarter strength the moment a pot shows crust. A pre-measured granule cannot be un-applied.

Never into the reservoir, and here is why

This is the single most common mistake in sub-irrigated growing, and the reason is mechanical rather than chemical. The reservoir is a standing body of water with no outflow. Fertilizer put into it has nowhere to go but up the wick, at a concentration that only rises as the water level drops and the dissolved salts stay behind.

The EarthBox system solves this with geometry rather than chemistry: a dry fertilizer strip is laid in a band on the surface, under a fitted cover, deliberately away from the water. Roots grow toward the band and take what they need. Nothing dissolves into the reservoir, so nothing concentrates in it.

That is the model to copy whatever pot you own. Feed the surface. Let the plant decide how much of it to use.

The flush is not optional in a reservoir pot

Every extension service that discusses salt buildup gives the same remedy, with figures. University of Maryland Extension says to pour a lot of water - at least twice the volume of the pot - onto the potting soil and let it drain completely, and to do it every four to six months. It also gives an instruction people get backwards: leach a plant before you fertilize, so you are not washing away the fertilizer you just added.

NC State Extension describes the same step for container plants, noting that when using liquid fertilizers, salt accumulation can sometimes be a problem and is evident by a white crust on top of the potting mix - and that the media should then be leached by slowly running water through it for several minutes.

In a self-watering pot that means lifting the inner pot out if your design allows it, or taking the whole thing to a sink or outside and top-watering it hard. It is the one piece of maintenance a reservoir pot genuinely needs that an ordinary pot does not, and it takes five minutes twice a year.

Liquid or controlled-release?

Both work in a reservoir pot. They fail differently, which is the useful way to choose between them.

Controlled-release granules have one advantage that matters here more than anywhere else. NC State Extension lists it first: avoidance of high initial salt levels in the growing media, plus availability of nutrients for several months and reduction of nutrient losses from the container by leaching and runoff. In a pot that cannot leach, “no initial salt spike” is close to the ideal property. The cost is control - once the granules are in the mix, the dose is set.

Liquid feeds are quick-release and water-soluble, cost less per unit of nitrogen, and suit plants already in growth. Their advantage in a reservoir pot is that you can halve the concentration the moment you see a crust, and halve it again next time. Their disadvantage is that they have to be remembered, which on a site built for people who forget to water is not a small objection.

If you water your reservoir pot and nothing else, take the liquid and put a reminder somewhere. If you would rather do one thing in spring and not think about it, take a replant kit or a controlled-release granule and accept that the dose is fixed.

When not to fertilize at all

Three cases, all published, and all of them cheaper than buying anything on this page.

A newly bought plant. Missouri Extension says new plants seldom need additional fertilizer for a few weeks. NC State adds that commercial mixes often contain a small amount of fertilizer for an early boost, though those nutrients usually last only a couple of weeks. If you have just repotted into fresh mix, you have already fed it.

Winter, or any time there is no new growth. Missouri is blunt: plants do not need fertilizer in winter when no new growth is apparent. NC State gives the calendar version - once a month from March to October, and not from November to February unless deficiency symptoms appear.

A plant that looks unwell. Missouri warns specifically against fertilizing to stimulate growth in poor conditions, because poor light or poor watering is more often the cause and the fertilizer may cause further injury. If a plant in a reservoir pot is struggling, the first thing to check is not nutrition - it is whether the reservoir ever runs down. That page covers it.

Brown leaf tips are the symptom to read correctly

The complaint that brings most people to a fertilizer page is browning at the tips and margins, and it points in both directions at once. UMD lists the symptoms of high soluble salts as browning or dieback of leaf tips and margins, reduced growth, lower leaf drop, dead root tips and wilting, with salt deposits visible on the outsides of pots. Missouri lists burned or brown leaf tips and margins under too much fertilizer.

So the same symptom means too much feed far more often than too little - and in a bottom-watered pot, more often still. Before you buy a fertilizer to fix brown tips, check the surface of the mix for a pale crust and the outside of the pot for deposits. If either is there, the answer is a flush and a weaker feed, not a stronger one.

The other common cause is the water itself rather than the fertilizer. Hard tap water deposits its own minerals, and in a pot that never drains those accumulate alongside the fertilizer salts. A TDS reading on your tap tells you which of the two you are dealing with.

The EarthBox question, answered

A steady share of the people who find this site are looking specifically for what to put in an EarthBox-style grow box instead of the branded kit. Here is the honest answer.

The branded replant kit is three things: pre-measured fertilizer, a pound of dolomite, and two reversible mulch covers. EarthBox publishes which analysis suits which crop - 7-7-7 Standard described as suiting all plants and especially fruiting and root crops, 8-3-5 Natural recommended for leafy crops. It also publishes the cover instruction: black side up, switching to white when peak temperatures stay above 85 degrees F.

The third-party kit does the same job for roughly two-thirds of the price and does not publish its analysis. That is a real trade rather than a technicality, because in a box you cannot flush, the analysis is the specification. If you buy it, feed conservatively and watch the surface.

The third route is to assemble it: a bag of dolomite, a fertilizer whose analysis you chose, and the cover you already own. That is the cheapest option by a distance and the one that gives you the most control. It also requires you to measure, which is the part the kits exist to remove. Our alternatives page covers the boxes themselves.

Dolomite, and the calcium problem nobody mentions

Dolomite appears in every EarthBox-style kit for a reason, and it is the reason Foliage-Pro is ranked first here. Dolomitic lime supplies calcium and magnesium and moderates pH, and the conventional way to apply it is to mix it into the growing medium before planting or work it into the surface afterwards.

A grow box gets replanted every season, so there is an obvious moment to add it. A houseplant in a reservoir planter does not. It may sit in the same column for three years, and the window for amending that column closed the day you planted it.

That leaves two options: a feed that carries calcium in solution, or a repot. The first is a bottle; the second is a weekend and a disturbed root ball. This is the whole argument for choosing a complete analysis over a cheaper NPK-only feed in this specific kind of pot, and it is not an argument that applies to an ordinary plant pot at all.

What we would buy

One houseplant in a reservoir planter: Foliage-Pro, at half the label rate, roughly monthly in growth and not at all in winter. The calcium is the reason.

A collection of houseplants, and a worry about overdoing it: the Espoma, at its published two teaspoons per quart every two to four weeks. The weakest analysis here is the most forgiving thing to get wrong.

A grow box of vegetables: the replant kit for the crop you are growing - 7-7-7 for fruiting crops, the 8-3-5 Natural for leafy ones - and leave the liquids alone. The pre-measured strip plus dolomite plus cover is a system, and substituting one part of it piecemeal is how it stops working.

Anyone who likes control: the 20-20-20 powder at a quarter of the extension-published rate, which works out to roughly a third of a teaspoon per gallon. Add dolomite at repotting time to cover the calcium it lacks.