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Self-watering planter from a plastic bottle

Turn a plastic bottle and a strip of old cotton cloth into a planter that waters itself by capillary action. Grow seedlings in it, and compare their growth with plants that are watered by hand.

  • Class 3 to 8
  • Biology
  • Best out of waste
  • Easy
  • 2 hours to build, then 2 weeks of growing
  • Free

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The science idea

Plants need water, but too little makes them wilt and too much drowns the roots, because roots also need air. Watering by hand often gives either a flood or a drought. A self-watering planter keeps a small supply of water in a reservoir and passes it to the soil slowly, as the soil dries. It is a clever use of an old plastic bottle and a good way to save water.

The movement of water up the wick is called capillary action. Water molecules stick to the walls of narrow gaps (adhesion) and to each other (cohesion). In a very narrow gap this pulls a column of water upwards against gravity. The gaps between the fibres of a cotton cloth are such narrow tubes. In a simple tube of radius r, the water rises to a height h = 2γcosθ ÷ (ρgr), where γ is the surface tension of water, θ is the angle of contact, ρ is the density of water and g is the gravitational field strength. The narrower the tube, the higher the rise. That is why paper towels, cloth and soil soak up water, and why a wick works best when it is short.

In the soil, the same effect passes the water between the soil grains from wet soil to drier soil. The roots take up the water, together with dissolved minerals. The water then travels up the stem to the leaves, where some of it is lost to the air as water vapour through tiny pores called stomata. This loss is called transpiration, and it helps to pull more water up the plant. So a plant that loses more water draws more from the reservoir; the system is self-regulating.

A wick can only feed the soil as fast as capillary action allows, so it is best for small plants such as seedlings, herbs, spinach or coriander, and not for big thirsty plants. You can make this into an experiment: grow identical seeds in three planters, one self-watering, one watered by hand every day and one watered every three days, and measure the height of the seedlings. You can also compare different wicks (cotton, nylon, wool) to see which carries water best.

Using a bottle that would otherwise be thrown away is itself an environment lesson: reduce, reuse and recycle. The planter also saves water, because less is lost by running off or evaporating from the soil surface.

Materials and tools

Cost: Free

A plastic bottle, an old cotton cloth, soil from the garden and a few kitchen seeds (methi or moong) are all free.

Materials
What you needHow manyFree substitute or where to find it
Plastic bottle, 1 to 2 litres, with its cap3 (1 for the planter, 2 for the comparison pots)A cold-drink or mineral-water bottle. It must be clean and must never have held pesticide, phenyl or any other chemical.
Strip of cotton cloth for the wick (old T-shirt, towel or cotton rope)1 strip, about 30 cm × 2 cmCut from old cotton clothes. Thick cotton usually carries water better than smooth nylon cloth.
Garden soil mixed with a little compost or sandAbout 6 cups (2 for each pot)From the garden or a pot. Sieve out stones.
Seeds that grow fast (methi, moong, coriander or mustard)A pinch for each pot (10 to 15 seeds, 30 to 45 in all)From the kitchen. Choose seeds that are whole and unbroken.
WaterAbout 500 mL to start, then about 1 litre more for the hand-watered pots over two weeksTap or rain water.
Tape or a rubber band1To hold the halves together.
Newspaper or dark paper1 sheetTo wrap the reservoir and stop algae growing in the light.
Cotton cloth, cotton wool or fine net to plug the refill hole1 small pieceA wad of old cotton cloth pressed into the hole, or a scrap of net held with a rubber band. Do not seal the hole airtight with clay or a cap.

Tools

  • Scissors and a craft knife (adult for the first cut)
  • Marker pen and ruler
  • An awl or a pointed tool for the refill hole and drainage holes (adult only; do not heat a nail on a flame)
  • Measuring cup or a syringe to measure water
  • Notebook

Safety first

Read this before you start

  • An adult should make the first cut in the bottle and the refill hole. Cut edges can be sharp: cover them with tape.
  • Use only a bottle that held water or juice. Never use one that held pesticide, cleaning liquid or any other chemical.
  • Wash your hands with soap after handling soil. Do not eat seedlings, soil or sprouts grown in a recycled bottle.
  • Keep the reservoir covered. Water standing in an open container can breed mosquitoes, so plug the refill hole with cloth or fine net, tape the two halves together with no open gap, and empty the reservoir and refill it with fresh water at least once a week.
  • Do not put the planter on a window ledge where it can fall, and keep it away from electrical sockets.

Parts and layout

These drawings show the parts and how they fit together. Keep them beside you while you build.

Scroll sideways to see the whole diagram.

Self-watering planter: how the bottle is cut and put togetherLeft: a plastic bottle with a dashed cut line. Middle: the two halves. The top half is turned upside down, neck down, and a strip of cotton cloth is pushed through the neck. The bottom half is the water reservoir with a small refilling hole. Right: the top half sits inside the bottom half so that the wick hangs in the water.Cut here (adult)top halfbottom halfSoil with seeds(in the top half)Neck points downtowards the waterCotton wick (cloth strip)pushed through the neckReservoir: water up tojust below the neckSmall refill hole with a plug
Figure 1. How the bottle is cut and put together The top half goes upside down into the bottom half. The wick hangs through the neck into the water.

Scroll sideways to see the whole diagram.

Capillary action: water rises higher in a narrower tubeThree thin tubes standing in water. Water climbs highest in the narrowest tube, less in a wider tube and least in the widest. The height is about inversely proportional to the radius. A wick works in the same way, because the gaps between its fibres are tiny tubes.riseWide tuberiseMedium tuberiseNarrow tubeThe narrower the tube, the higher the water climbs (not to scale).
Figure 3. Capillary action: narrow tubes lift water higher The gaps between the fibres of a wick work as extremely narrow tubes.

Step-by-step build

12 steps. Do them in order, and test each part before you go on to the next.

  1. Wash the bottle and mark the cut

    Wash the bottle and take off the label. Draw a line all the way round the bottle about halfway up the bottle, on the straight part below the sloping shoulder. Check it is level with a ruler.

    Tip

    Wrap a piece of paper round the bottle, line up the edges, and draw along the edge of the paper for a straight line.

  2. Cut the bottle in two

    An adult makes a small cut on the line with a knife. Then cut right round with scissors. You have two parts: the top with the neck, and the bottom with the flat base. Cover the cut edges with tape.

    Common mistake

    Cutting at a slant. The two halves then do not sit level on each other.

  3. Make the refill hole

    In the side of the bottom part, about 3 cm below the cut edge, an adult makes a hole about 1 cm wide by pushing in the point of an awl or a small knife and widening it with the tip of the scissors. This is where you add water. You will plug it with a wad of cotton cloth or a piece of fine net, which keeps mosquitoes out but lets air in as the water level falls.

    Tip

    Make the hole on the side that will face the wall so the plug does not catch.

  4. Make the wick

    Cut a strip of cotton cloth about 30 cm long and 2 cm wide. Fold it lengthways in half, or twist it into a rope. Wet it so that it is easy to handle.

    Common mistake

    Using a thin thread or nylon cloth. It does not take up much water. Use thick cotton.

  5. Thread the wick

    Remove the cap from the top part, turn it neck-down, and push the strip through the neck so that about 10 cm hangs out below the neck and the rest lies in the cone above.

    Tip

    Tie a knot at the top end of the strip or spread the top end out so that it does not slip down through the neck.

  6. Fill the soil

    Spread the top end of the wick out in the cone. Fill the cone slowly with soil mixed with a little sand or compost, pressing lightly with your fingers until it is about two thirds full. Do not pack it hard.

    Common mistake

    Packing the soil too tight. Water and air cannot move through packed soil, and the roots cannot grow.

  7. Fill the reservoir

    Pour water through the refill hole into the bottom part, about 300 to 500 mL for a 1.5 litre bottle. The water level should be about 2 cm below the tip of the neck of the top part when the planter is put together.

    Tip

    Check with a ruler or stick through the refill hole after you fit the halves together in the next step. The wick must dip into the water, but the neck and the soil must not sit in it. Pour out a little water if the level is too high.

  8. Put the planter together

    Push the top part, neck down, into the bottom part so that the wick hangs into the water. Tape the two halves together with several strips. Wrap the bottom part in paper to keep out light, and plug the refill hole.

    Common mistake

    Letting the neck sit in the water so that the soil becomes a swamp. Leave an air gap of about 2 cm between the neck and the water.

  9. Wet the soil and sow

    Water the soil from the top once, gently, until it is damp (not dripping), so the wick starts working. Sow 10 to 15 seeds about 1 cm deep, and cover them lightly with soil.

    Tip

    The first watering from the top starts the capillary flow. After that the wick does the work.

  10. Prepare two comparison pots

    Cut two more bottles and use only the bottom part of each as a plain pot with no wick: an adult makes three or four small drainage holes in the base, and each pot stands on a saucer or lid. Fill them with the same soil and sow the same number of seeds. Water one by hand with 50 mL a day and the other with 50 mL every three days. Put all three pots in the same place.

    Common mistake

    Using different seeds or soil. For a fair test, only the way of watering should differ.

  11. Measure every two days

    Every two days (days 2, 4, 6 and so on) at the same time, note the soil colour and feel, the height of the tallest seedling in millimetres, the number of leaves, and how much the water in the reservoir has fallen. Write them in your table.

    Tip

    Measure from the soil surface to the tip of the tallest leaf with a ruler held beside the stem.

  12. Compare and conclude

    After 14 days, draw a line graph of height against day for the three pots. Say which pot grew best and why. Note how much water each pot used.

    Tip

    If the wick pot did worse than the others, check whether the wick was in the water and whether the soil was too wet.

How it should look when it is finished

Finished model

A tidy plastic planter about 20 cm tall: a clear or paper-wrapped lower part that holds water, and an upper cone filled with dark soil, with a tuft of green seedlings growing from the top. A small plug marks the refill hole, and a thin strip of cotton cloth can be seen hanging from the neck into the water.

This is a description, not a photograph. Compare your own model with it and with the figures above.

How it works

The bottom half holds the water, and the top half holds the soil. The wick links them: one end sits in the water and the other spreads through the soil. Because water sticks to the cotton fibres and to itself, it creeps up the wick through the tiny gaps between the fibres, just as it creeps up a narrow tube.

When the soil is dry, water leaves the wick and moves into it. When the soil is damp, the wick has nowhere to send water and its flow slows right down. So the soil is kept evenly damp instead of being flooded or dried out, and this is much better for the roots.

The roots take up water, and it moves up the stem to the leaves. Water vapour leaves through pores in the leaves (transpiration), which pulls more water up from the roots. The water in the reservoir slowly falls, and you only have to top it up through the refill hole.

Scroll sideways to see the whole diagram.

Self-watering planter: how the water reaches the plantA cross-section of the finished planter. Water in the reservoir climbs a cotton wick by capillary action, spreads through the soil, and is taken up by the roots of the seedlings. Leaves lose water vapour to the air. The wick only takes up as much as the soil needs, so the plants are not drowned.1234What happens, step by step1Water climbs the wick bycapillary action.2It spreads through the soiland keeps it damp.3Roots take up the waterand minerals.4Leaves lose water vapour(transpiration), so morewater is drawn up.SeedlingsReservoirwater vapour
Figure 2. How the water reaches the plant Capillary action in the wick, spreading in the soil, root uptake and transpiration make a continuous stream of water.

Troubleshooting

If something goes wrong
What you seeLikely causeWhat to do
The soil stays dryThe wick does not touch the water, is too thin, or is not touching the soil.Lengthen the wick, use thicker cotton, and check that the lower end is in the water and the upper end spreads in the soil.
The soil is very wet and smells badThe neck or soil sits in the water, or there is too little air.Lower the water level so that there is a gap, and loosen the soil.
Seeds do not sproutOld seeds, buried too deep, or soil too dry or too cold.Use fresh seeds 1 cm deep, and give the first watering from the top.
Green slime or algae grows in the reservoirLight is reaching the water.Wrap the reservoir with paper or paint it, and change the water.
Mosquito larvae in the waterThe refill hole was left open, or there is a gap between the two halves.Plug the hole with cloth or fine net, close any gap with tape, empty the reservoir and refill it with fresh water. Change the water at least once a week.
The two halves fall apartThe halves are not tight.Tape them together with several strips, or use a rubber band.

Test and record

Measure the tallest seedling in each pot at the same time every day. Write the water used from the reservoir (in mL) in the last column for the wick pot.

Table: growth of seedlings in the three pots
DayHeight, wick pot (mm)Height, watered daily (mm)Height, watered every 3 days (mm)Leaves (wick pot)Water used (mL)
2
4
6
8
10
12
14

Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.

Viva questions and answers

A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.

  1. What is capillary action?

    The rise of a liquid in a very narrow tube or gap, caused by the adhesion of the liquid to the walls and the cohesion between its own molecules.

  2. Why does the wick not flood the soil?

    Water moves into the soil only as fast as the dry soil pulls it. When the soil is damp, the flow slows, so the soil stays damp but not wet.

  3. What is transpiration?

    The loss of water vapour from the leaves, mainly through the stomata. It helps to pull water up through the plant.

  4. Why does the plant need air around its roots?

    Roots respire and need oxygen. In waterlogged soil there is little air, so the roots can rot.

  5. Which wick materials would you test and what would you expect?

    Cotton, wool and nylon. Cotton and wool absorb water well, so I would expect them to do well, and smooth nylon cloth to carry less. The test then shows whether that is true.

  6. Why wrap the reservoir in paper?

    Light lets algae grow in the water. Paper blocks the light.

  7. How does this project help the environment?

    It reuses a plastic bottle and it saves water, because less is lost by run-off and evaporation.

  8. How did you make the test fair?

    Use the same soil, seeds, light and position for all three pots, and change only the way they are watered.

Free ebook: Viva Questions for Science Projects

Written report outline

Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.

Aim
To make a self-watering planter from a plastic bottle and to compare the growth of seedlings in it with plants watered by hand.
Apparatus
Three plastic bottles, a cotton cloth wick, soil, seeds, water, scissors, tape, a ruler and a measuring cup.
Procedure
Describe how you cut the bottle, made the wick and refill hole, filled the soil and the reservoir, planted the seeds, and how you measured the seedlings every two days. Include the diagrams.
Observation
The table and the graph of seedling height against day for the three pots.
Result
After 14 days the seedlings in the wick pot were ... mm tall, in the daily-watered pot ... mm and in the pot watered every three days ... mm. The wick pot used ... mL of water.
Conclusion
A cotton wick draws water up by capillary action and keeps the soil evenly damp, so seedlings grow well with less effort and little waste of water.

Free ebook: Writing a Science Project File

Take it further

  • Test different wicks (cotton, wool, nylon, jute rope) and measure how high the water climbs in a fixed time.
  • Put a coloured dye or a few drops of food colour in the reservoir and watch it climb up the wick and into the soil.
  • Measure the water used per day by the wick pot on a sunny day and on a cloudy day.
  • Grow the same seeds in the light and in the dark and compare.
  • Make a bigger planter from a clean 5 litre water can (never one that held oil, paint or pesticide), with two or three wicks, and grow a chilli plant. Does one wick give it enough water?

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