If you ask 10 different gardeners that question, you will likely get 10 different answers. Who is right and who is wrong? I had a long-standing difference of opinion with a friend on the answer to this very question, and so I set out to get to the rocky (or not rocky) bottom of this!
The myth is, gravel or rocks in the bottom of a plant pot will improve drainage. This is false.
Do not put rocks in the bottom of plant pots. Putting gravel or rocks in your plant pots with drainage holes does NOT improve drainage, it instead increases the water saturation zone that leads to root rot. There are exceptions, read on to learn when you can and when you should use rocks in the bottom of your planter.
When To & Not To Use Rocks In A Plant Pot!
- Do NOT put rocks in the bottom of your pot if:
- you do not need the extra weight
- you do have drainage holes
- you have common pot shape designs
- if you do not have any of the reasons why TO add rocks in your planter
- Put rocks in your planter if:
- you need the extra weight in the bottom to prevent tipping over in the wind
- there are no drainage holes in the bottom of your pot (see below for details)
- the design of the pot is very tall and narrow, greater than 2:1
What To Put In The Bottom Of Your Planter For Better Drainage:
For most of your pots put one of these over the drainage holes to prevent the holes from clogging to ensure drainage:
For Most Pots with Drainage Holes:
- landscape fabric
- coffee filter
- clean steel wool
- a sponge
- window screening
- recycled clean strainer
- coat hanger with pantyhose stretched over it
For pots that-
-you need the extra weight to anchor the pot
-no drainage holes
You can fill up to 12-24″ from the top, what to fill large plant containers with:
- cut up pool noodles
- empty crushed water bottles
- lava rock
- styrofoam peanuts
- upside down smaller pots
- drainage tubing
- used potting soil
When people do use rocks in the bottom of their container and are asked why the usual answer is a variation of one of the following:
- “So the potting soil does not wash out of the pot
- “For drainage”
- “My pot is so large, the roots don’t get down there anyway, why use potting soil?”
- “My grandma always did, and her plants thrived”
Here is why rocks do not add drainage to pots, for those who want to know WHY…
First, and yes this is an always, use potting soil, not garden soil. This Youtube Short will show you why.
Next, think of potting soil like a sponge. When completely dry, it is hard to get wet again. However, when damp, you can add water and it will hold more. When saturated, any more water dripped in the top will simply move through and push water out the bottom, which will not hold anymore.
What happens when you stand that saturated sponge on its end, so it is tall? In a moment, the top is drier than the bottom. The bottom will still be saturated while the top is only damp.
One great reason to check the bottom before watering, but I digress. Now think of a sponge the same size and shape to fit inside your pot. How much water can it hold? At what zone does the sponge shift from moist at the top to saturated at the bottom? Remember your potting soil acts a lot like a sponge and most plants like their roots to be moist but not saturated. Saturated soil encourages root rot and quite literally drowns the plant. Read about this in the article ‘Do Plants Need Saucers?’ how and why click here.
What happens when you put rocks in the bottom of your planting container?
Now take that same sponge, cut an inch or 2 off the bottom. How much water does the sponge hold now? Less. The zone where the sponge is not moist but saturated? Closer to the top of the sponge than it was(where the roots are). Gravel does not hold any water, of course. However, neither does it draw the water out of the finer soil, (draining it), either. (Has anyone tried toweling off with a chunky knit sweater rather than a towel lately?)
This all has to do with soil mechanics and capillary action and cohesion and adhesion and gravity, but the sponge example illustrates it really well. You don’t have to get bogged down in scientific jargon to have that ah-ha moment.
Why did my Grandma put rocks in the bottom of her containers then?
The same reason why different sizes and shaped containers can work just fine, … It is not that you can’t use them, just the watering schedule may be different. It is not that you can’t put a layer of gravel at the bottom, it’s just you won’t have to, and there are times it is just easier to care for your plant if you didn’t.
When You DO Want Rocks In The Bottom & Why.
- Sharply draining soil
- Tall narrow planter
- Cache pot without holes
When you go look at terra cotta pots from a retailer with a good selection, you will see standard pots and some called ‘azalea’ pots. (I know this sounds off-topic, but bear with me.) The azalea pots are roughly ¾ as tall as wide, whereas standard pots are as tall as wide. Azalea pots are a standard shape pot because they are used in the production of….azaleas ( I know..shocker!). Azaleas are grown in a very gritty, exceptionally well-drained mix that does not hold water as well as a sponge.
Hence no need for extra depth to keep roots out of the saturated zone, and no need for the expense of extra soil.
Another reason you would want rocks at the bottom is if you have a large container that’s designed to have a smaller bottom than the top, or simply a really tall and skinny pot. You need the extra weight to prevent tipping over in the wind, especially if you are growing large, bold-leaved plants that would catch the wind like a sail.
Tall narrow designs just have that saturated water level WAY far below where annual roots will get to, and there is no reason why you need to invest in potting soil ALL the way down. You can use gravel but, if you do not need the extra weight for stability, use cut-up swimming pool noodles, crushed empty water bottles, and much less expensive bark mulch….You have the idea, use what you have that will not disintegrate with water, or harm your potting soil. Then cover with a layer of landscape fabric to keep your potting soil where it needs to stay. In the root zone of your plants.
You do need a nice substantial layer of rock ( or one of the above) if you don’t have drainage holes at the bottom. (Yes you want drainage holes, but I know some of you are going to use cache pots without holes to plant in any way.) The substantial layer on the bottom gives a place for the excess water to go. (If there is no excess water, you’re not watering enough volume when you water. This sounds counter-intuitive, I know, but another future post will cover this in-depth). VERY IMPORTANT, put a layer of landscape fabric cut to size between the potting soil and the gravel. This is to prevent the soil from simply filtering down into the gravel canceling out the effort of gravel in the first place. Just do it, you will thank yourself later.
A great way to use cache pots without holes is to layer gravel on the bottom and drop in fully planted inexpensive, non-decorative pots into them to sit on top of the gravel. Easy to swap out for new plantings, easy to fix too-much standing water (pull out the inner pot, dump out or siphon out excess water, and pop pot with plants back in.)
When you do not want rocks and why.
When you have typical plants in typical pots where you want the potting soil to hold as much moisture in reserve as possible, delaying when you need to water again. ( If you do not want to be a slave to watering all summer, the larger the pot the better, and a restrained hand in the volume of plants in that container, again more details on that in another post All You Need To Know About Watering, there is so much to cover, whew!)
How do I keep the potting soil from washing out of my container? OR, How to Cover Drainage Holes in Pots?
A very good question. There have been many great creative solutions to this challenge. One involves a piece of crockery. I know sounds like the gravel, but no, read on. One piece of broken crock ( a broken pot such as terra cotta) was placed over the hole curved side up. This allows the water to still exit the drainage hole, without taking a large volume of potting soil with it.
Another good solution is to lay a piece of landscape cloth (or a coffee filter, cheesecloth, or any water resilient straining material) over the drainage hole and fill the pot with potting soil.
What about containers without drainage holes?
See above “When you want rocks in the bottom and why.”
Putting rocks on top of potted plant soil.
Putting rocks on top of a potted plant’s soil is a perfectly acceptable method to cover the soil decoratively. You might have to check if your plant needs water differently, since you can’t see, or may not easily be able to touch the potting soil. If your plant and pot are light enough to pick up, one way to judge watering is the weight. Feel how heavy it is immediately after watering thoroughly. Water is heavy, ever carry a gallon of water around? That is not insubstantial. The phrase I like to use when describing if a pot is ready for watering, with a plant that prefers to dry down a little between waterings is ‘alarmingly dry’. You won’t see wilting leaves or other signs of distress, but when you pick up the planter you think, “Wow, that is alarmingly dry”.
A layer of a product called Gnatnix will control gnats, those pesky ‘fruit-fly looking’ pests that can be problematic on potted plants. ( However, if you have gnats, they are a warning sign you might be overwatering your plant. Gnats love those conditions… )
Lava Rock for Plant Drainage?
Yes, you can use lava rock, however, remember it is very lightweight, and one of the reasons why you would want to add rocks or gravel if to ADD weight. If you don’t want to add weight, try some of the inexpensive or free suggestions above and skip the expense of lava rock. If you have extra from another project handy anyway, sure, use it, as long as it hasn’t been treated with anything harmful to plants.
After transplanting trees, shrubs, annuals, and perennials, into your landscape, watering will be first on your list of concerns. Getting this right will solve the number 1 reason why plants
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