Watering Container Plants: The Finer Points By Paula Szilard How often have you heard someone say, “Let me just throw a little water on these plants and I’ll be right there.” If you actually do this—throw water on plants, that is, they can’t be doing all that well, or if they are doing ok in spite of it, they could be doing a lot better. Improper watering, either too little or too much, is the primary cause of plant mortality. Watering is part science and part art. The roots are totally confined, making the plant utterly dependent on your expert watering and feeding. Water needs will vary, of course, depending on the plant and the conditions. If it’s located near a heating vent, of course it will need more water in the winter. Ditto if it’s in sun rather than in shade. Pot up Plants with Watering in Mind. The way you pot up a plant will determine whether watering will be easy or difficult and the material the pot is made of and its size will have an influence over how often you need to water. Make things easy on yourself and leave an inch or an inch and a half of space on top. That way you can fill that space with water and it will gradually seep down and evenly water the entire pot. Plants potted up in clay pots are notorious for drying out faster, than, say plastic or glazed ceramic ones. On the other hand, cacti and succulents are often potted up in clay pots for this very reason. While it’s always best to use the correct size pot—not too large and not too small, it is clear that smaller pots, even plastic, will dry out more rapidly. By the way, plants in pots do not need any coarse textured material on the bottom. This has not been shown to impede drainage. What they need is something to keep the potting mix from falling through the drainage holes. That could be a small piece of an old window screen or a curved pottery shard. Test Moisture Levels. Experienced, knowledgeable growers can often tell by lifting a pot how dry it is. For many of us, though, such judgments carry some risk. The time honored method of sticking a finger into the soil mix will only get you so far, especially if the pot is filled with a heavy cactus mix or if the plant is large. So, get yourself some moisture meters and put them in convenient locations near your plants. It’s especially important for large plants, like that ancient Christmas cactus you inherited! Moisture meters have a downside. If your mix is too high in dissolved minerals, they may not work properly. Repotting is the recommended remedy for this problem. For the very largest plants, some interiorscapers use a soil probe. This instrument actually reaches deep into the pot and brings up a small amount of soil, so you can judge the moisture level by touch and sight, since dry organic matter tends to be lighter in color. Be Attentive to Drainage of Potting Mixes While some potting mixes retain a lot of moisture, others retain almost none. The more organic material a mix contains, the more moisture it retains. If the label has vermiculite on it, the mix will also retain more water. Generally the peat based mixes retain more, but even these need some material for drainage, usually perlite. Potting mixes which have coconut fiber (coir) also retain more water. There is a highly water retentive mix containing coconut fiber manufactured by Miracle Grow for outdoor use. Do not use this on any plants you later plan to bring inside. It will retain too much water and the plants may not survive. Plain sphagnum peat of the type used to pot up the orchids you buy at the grocery store retains so much water, these plants will likely not survive if they are not repotted into a well drained mix. When you start adding perlite, sand and pebbles, it speeds up drainage. Mixes formulated for cacti and succulents drain with lightening speed and retain very little moisture. Make Sure the Top is Level. Sometimes careless and uneven watering results in the redistribution of the potting mix at the top of the pot. There might be “valleys” and “peaks”. If you water without evening out the surface, then you will only be watering part of the root system. Say, you have 4 stems of a dracaena in a large pot and the soil around the front two is lower. Where do you think the water is going to flow? The back stems will get little or none and the front stems will be supersaturated. Over time, the stems on higher ground will do poorly and possibly die out. Either use a rake from a small child’s gardening set or a large fork to even out the surface periodically and while you’re doing that, be sure the pot is sitting on a level surface. Pay Attention to Seasonal Changes. This is obvious, but the more forgetful among us need to remind ourselves to reduce the amount of water plants get in the darker, shorter days of late fall and winter and increase the amount of water during the spring, summer and early fall. There is a major exception to this rule. Plants near heating vents may need more, not less water in the winter. By extension, a plant’s location in your sunroom or enclosed porch will also influence the amount of water it uses. The more sun it gets the more it photosynthesizes and the more it photosynthesizes, the more water it uses. Water Slowly and Around the Edges. Water slowly and cover the entire surface of the pot. The easiest way to do this is to water both around the perimeter and in the center. This way you are sure to water the entire root system. Water Until it Runs Out. Apply enough water, spread evenly on the top of the planting mix, so that about 25% runs out of the bottom of the pot. Then don’t water again until the mix tests dry. The needle on the moisture meter should be in the bottom 1/3. Some plants, of course do not like to dry out this much—African violets and begonias, for instance. You only want the surface of the mix to dry out a bit. Never Apply Water at an Angle or at High Pressure. This is especially true of watering with a hose end sprayer. Water from the top down if you can. If you can’t, then water gently in the areas where there is exposed soi1. If your plant is so root bound that there is no exposed soil, water from the bottom. If you apply water at an angle or at high pressure, especially with a hose, you will likely rearrange the surface of the terrain to create hills and valleys. This will result in uneven watering later. Also if you water at an angle with a hose end sprayer, much of the water will probably wind up outside of the pot and so will some of the potting mix. Mostly, Don’t Leave Water in the Saucer. It’s rarely wise to leave a saucer full of water when you’ve finished watering. In general, if the pot is in a saucer, the drainage water should be emptied or sponged up after an hour or so. There are, however, several instances when you can leave water in the saucer without causing harm. You may need to water from the bottom because growth is so dense that the water runs right off your plant. Perhaps your plant needs repotting and the roots are so tightly packed into the pot that water doesn’t penetrate. Also, you may have very thirsty plants, such as Angel’s Trumpets (Brugmansias) out in the sun during the intense summer heat. You would otherwise be watering these plants twice a day. Leaving the saucers full of water is one way to give them a little reserve. Don’t Overwater. Overwatering means watering a plant that is already moist. DO NOT DO THIS! It is the fastest way to kill your plant! The plant roots need oxygen, which they cannot get when the mix is soggy. Repeated watering of an already moist plant will rot out the roots. With the rotted roots, the plant cannot absorb water and will die of thirst! Remember that a plant can wilt from being too dry (and in need of water) or from being too wet (and the rotting roots cannot transport water that is already present). Always make sure a wilting plant shows symptoms of soil that is too dry or a pot that is too lightweight before watering. Wet Wilt Wet wilt occurs when there is adequate soil moisture, yet roots cannot absorb water fast enough to compensate for the rate of water lost from the leaves (transpiration). This is commonly seen in melons and cucumbers in the garden but can also happen to plants in containers: very large leaves wilting on a sunny day, even though the soil is moist. Low humidity and wind can exacerbate the situation. Perhaps the best solution here is to provide shade or raise the humidity temporarily with a light mist on the leaves. The already moist or wet soil does not need water. Author’s Notes: I would like to thank my sister Irma Kristel, who has a small interiorscape business for her many helpful suggestions.

Watering Container Plants: The Finer Points

Paula Szilard

How often have you heard someone say, “Let me just throw a little water on these plants and I’ll be right there.” If you actually do this—throw water on plants, that is, they can’t be doing all that well, or if they are doing ok in spite of it, they could be doing a lot better. Improper watering, either too little or too much, is the primary cause of plant mortality.

Watering is part science and part art. The roots are totally confined, making the plant utterly dependent on your expert watering and feeding. Water needs will vary, of course, depending on the plant and the conditions. If it’s located near a heating vent, of course it will need more water in the winter. Ditto if it’s in sun rather than in shade.

Pot up Plants with Watering in Mind.

The way you pot up a plant will determine whether watering will be easy or difficult and the material the pot is made of and its size will have an influence over how often you need to water. Make things easy on yourself and leave an inch or an inch and a half of space on top. That way you can fill that space with water and it will gradually seep down and evenly water the entire pot. Plants potted up in clay pots are notorious for drying out faster, than, say plastic or glazed ceramic ones. On the other hand, cacti and succulents are often potted up in clay pots for this very reason. While it’s always best to use the correct size pot—not too large and not too small, it is clear that smaller pots, even plastic, will dry out more rapidly. By the way, plants in pots do not need any coarse textured material on the bottom. This has not been shown to impede drainage. What they need is something to keep the potting mix from falling through the drainage holes. That could be a small piece of an old window screen or a curved pottery shard.

Test Moisture Levels.

Experienced, knowledgeable growers can often tell by lifting a pot how dry it is. For many of us, though, such judgments carry some risk. The time honored method of sticking a finger into the soil mix will only get you so far, especially if the pot is filled with a heavy cactus mix or if the plant is large. So, get yourself some moisture meters and put them in convenient locations near your plants. It’s especially important for large plants, like that ancient Christmas cactus you inherited! Moisture meters have a downside. If your mix is too high in dissolved minerals, they may not work properly. Repotting is the recommended remedy for this problem. For the very largest plants, some interiorscapers use a soil probe. This instrument actually reaches deep into the pot and brings up a small amount of soil, so you can judge the moisture level by touch and sight, since dry organic matter tends to be lighter in color.

Be Attentive to Drainage of Potting Mixes

While some potting mixes retain a lot of moisture, others retain almost none. The more organic material a mix contains, the more moisture it retains. If the label has vermiculite on it, the mix will also retain more water. Generally the peat based mixes retain more, but even these need some material for drainage, usually perlite. Potting mixes which have coconut fiber (coir) also retain more water. There is a highly water retentive mix containing coconut fiber manufactured by Miracle Grow for outdoor use. Do not use this on any plants you later plan to bring inside. It will retain too much water and the plants may not survive. Plain sphagnum peat of the type used to pot up the orchids you buy at the grocery store retains so much water, these plants will likely not survive if they are not repotted into a well drained mix. When you start adding perlite, sand and pebbles, it speeds up drainage. Mixes formulated for cacti and succulents drain with lightening speed and retain very little moisture.

Make Sure the Top is Level.

Sometimes careless and uneven watering results in the redistribution of the potting mix at the top of the pot. There might be “valleys” and “peaks”. If you water without evening out the surface, then you will only be watering part of the root system. Say, you have 4 stems of a dracaena in a large pot and the soil around the front two is lower. Where do you think the water is going to flow? The back stems will get little or none and the front stems will be supersaturated. Over time, the stems on higher ground will do poorly and possibly die out. Either use a rake from a small child’s gardening set or a large fork to even out the surface periodically and while you’re doing that, be sure the pot is sitting on a level surface.

Pay Attention to Seasonal Changes.

This is obvious, but the more forgetful among us need to remind ourselves to reduce the amount of water plants get in the darker, shorter days of late fall and winter and increase the amount of water during the spring, summer and early fall. There is a major exception to this rule. Plants near heating vents may need more, not less water in the winter. By extension, a plant’s location in your sunroom or enclosed porch will also influence the amount of water it uses. The more sun it gets the more it photosynthesizes and the more it photosynthesizes, the more water it uses.

Water Slowly and Around the Edges.

Water slowly and cover the entire surface of the pot. The easiest way to do this is to water both around the perimeter and in the center. This way you are sure to water the entire root system.

Water Until it Runs Out.

Apply enough water, spread evenly on the top of the planting mix, so that about 25% runs out of the bottom of the pot. Then don’t water again until the mix tests dry. The needle on the moisture meter should be in the bottom 1/3. Some plants, of course do not like to dry out this much—African violets and begonias, for instance. You only want the surface of the mix to dry out a bit.

Never Apply Water at an Angle or at High Pressure.

This is especially true of watering with a hose end sprayer. Water from the top down if you can. If you can’t, then water gently in the areas where there is exposed soi1. If your plant is so root bound that there is no exposed soil, water from the bottom. If you apply water at an angle or at high pressure, especially with a hose, you will likely rearrange the surface of the terrain to create hills and valleys. This will result in uneven watering later. Also if you water at an angle with a hose end sprayer, much of the water will probably wind up outside of the pot and so will some of the potting mix.

Mostly, Don’t Leave Water in the Saucer.

It’s rarely wise to leave a saucer full of water when you’ve finished watering. In general, if the pot is in a saucer, the drainage water should be emptied or sponged up after an hour or so. There are, however, several instances when you can leave water in the saucer without causing harm. You may need to water from the bottom because growth is so dense that the water runs right off your plant. Perhaps your plant needs repotting and the roots are so tightly packed into the pot that water doesn’t penetrate. Also, you may have very thirsty plants, such as Angel’s Trumpets (Brugmansias) out in the sun during the intense summer heat. You would otherwise be watering these plants twice a day. Leaving the saucers full of water is one way to give them a little reserve.

Don’t Overwater.

Overwatering means watering a plant that is already moist. DO NOT DO THIS! It is the fastest way to kill your plant! The plant roots need oxygen, which they cannot get when the mix is soggy. Repeated watering of an already moist plant will rot out the roots. With the rotted roots, the plant cannot absorb water and will die of thirst! Remember that a plant can wilt from being too dry (and in need of water) or from being too wet (and the rotting roots cannot transport water that is already present). Always make sure a wilting plant shows symptoms of soil that is too dry or a pot that is too lightweight before watering.

Wet Wilt

Wet wilt occurs when there is adequate soil moisture, yet roots cannot absorb water fast enough to compensate for the rate of water lost from the leaves (transpiration). This is commonly seen in melons and cucumbers in the garden but can also happen to plants in containers: very large leaves wilting on a sunny day, even though the soil is moist. Low humidity and wind can exacerbate the situation. Perhaps the best solution here is to provide shade or raise the humidity temporarily with a light mist on the leaves. The already moist or wet soil does not need water.

Author’s Notes:

I would like to thank my sister Irma Kristel, who has a small interiorscape business for her many helpful suggestions.