RHS research shows the best way to water hanging baskets

Hanging baskets are notoriously difficult to water. A recent RHS survey found three quarters of respondents wanted advice on how to water baskets and containers. Janet Manning, former RHS Water Management Specialist, trialled a variety of watering methods to find the best one.

Hanging basket watering techniques trial 

The climate is changing and gardeners are having to adapt to the less predictable extremes of weather such as the cold spring of 2018 and the long drought through the following summer.  The winter of 2019/2020 saw many gardens flooded, and we know gardeners would like updated watering advice to cope with these extremes.

A trial by the RHS Water Management Specialist Janet Manning during the summer of 2019 looked at 10 different ways of watering hanging baskets to get a better understanding of what we need to do to keep petunias looking good. With this in mind, watering methods were trialled, not to test the systems per se, but to better understand the effect of different applications and soil moisture contents on the quality of plants produced. Results were applicable for the hanging basket trial but the advice can apply to larger scale planting as well.

Headline results  

The table shows the different applications used and how they were expected to affect growth of petunias.

Results 

1. The most efficient system in terms of the number of flowers produced per litre of water applied was the self-watering and low dosed, manually watered baskets. This is because they also had the least wastage draining from the bottom of the basket, the self-watering baskets capture the drainage in their reservoir and there is less run-through using smaller doses of irrigation for manually watered baskets.
2. It’s really hard to water hanging baskets without the water running out of the bottom.  The only basket that solved that problem was the self-watering, wicking system.  As the water was not delivered to the surface but directly into the reservoir, the leaves stay dry, and if it rains, the excess collects and is available to the roots rather than draining away.  As a result it wasted less than 7% of the water applied to it, and a good proportion of that was excess rain that had overflowed from the reservoir.
3. The soil moisture content has an influence on the amount of water that is lost by drainage.  So when the compost was really dry, (SMFC and SUB), as much as 42% of the water applied ran through or off the surface of the media.  Conversely, when the compost is kept wetter (SOL and TD Hi), 30-70% of the applied water was lost.
4. Doubling the amount of water applied did not double the amount of flowers produced.  Only 8-15% more flowers were produced when the water volume applied was doubled for manually watered baskets.  The self-watering basket (SW) produced a similar number of flowers as the manually watered high rate basket (TOPM Hi), but it received about a third less water.  This was not intentional, but the water was added to the reservoir which, at times, was not large enough to hold the full high rate dose intended.
5. The soil moisture sensor should have maintained the baskets at a reasonable moisture content but following the manufacturers recommendation of attaching the dripper to the back of the probe was probably not the best way to operate it in a small container.  Less than optimum volumes were delivered as the water reached the sensor very quickly and switched off the flow before the whole basket was watered. 
6. When people were asked how much water they thought the basket could hold, most had no idea and several admitted that it hadn’t occurred to them to consider it.  Estimates varied hugely and only some of the most practical horticulturists were close to guessing 3.4 litres of water holding capacity in a 5 litres container, which is equivalent to 68% soil moisture by volume.  They also had no idea how much water the plants would need, confirming the need to improve the advice on how much and how often we should be watering.  The field capacity of three baskets was measured at the end of the trial and found to be similar to the fresh compost.  The 5 litres basket was found to hold 3.4 litres water at the start of the trial and ranged 3.05-4.19 litres at the end of the trial, so the compost has not degraded significantly and in some cases improved, maybe due to microbial activity.
7. The irrigation volumes were only increased once to cope with increased water use from the increased growth of the plants, but soil moistures dipped to below 20% v/v in all baskets at some point during the trial which probably affected flower production.  Best flower production was achieved in manually watered and self-watered baskets, possibly due to more consistent moisture through the root zone, although this was not measured.
8. Overall, after 9 weeks since planting, and surviving the hottest July day on record (36.7^oC at RHS Garden Wisley) every one of the baskets used less than 25 litres, 4 watering cans full (7.5 litres each) of water over the 9 weeks in total, with most baskets maintaining a floral display. An average over the whole trial of 142-380ml per day.  This confirms Tijana Blanusa’s earlier work that demonstrated that hanging baskets could survive on 160ml per day.  

Conclusions

• Soil moisture content is really important to maintain flowering and growth, but is also important in reducing wastage and capturing water rather than allowing it to become surface runoff. Very dry and very wet conditions just cause more water to be wasted.

•  Irrigation systems automate the delivery of the water, saving time and labour, but they still need intervention to maintain and optimise.  They are not a 'fit and forget' solution, and if treated as such are likely to perform worse than a manually watered basket.
• The trial results can be used to describe the summer and winter differences in the water cycle at a scale that many gardeners can relate to, ie when there are winter rains, the water is best captured in below-ground reservoirs to be used later.  Having the surface of the compost bone dry isn’t a problem, and probably helps to reduce evaporation from the soil, reducing water loss.  When the compost is very wet, plants help to dry the compost by evapotranspiration, making space in the root zone to capture the next rain when it comes.
• Gardeners should aim to achieve a soil moisture deficit or to ‘keep the glass half full’, so that there is sufficient moisture for the plants to grow but also sufficient air space for roots to respire and to capture rain, rather than allowing it to drain away.
• The questionnaire results confirmed that very few gardeners, even the expert ones, have given any thought to how much water their soil is capable of holding, and therefore how much might be draining away out of sight.  This confirms the need for training expert gardeners as well as improving advice to members and wider horticultural community.

10 things gardeners can do differently

1.  Slow the flow when you are watering and don’t be in a rush to get the water to soak into the compost, it will run off the surface or drain through the bottom of the container, where it will be wasted.
2. Using double the amount of water also doubled the amount of water wasted, but produced only 8-15% more petunia flowers.
3. 30cm baskets with 6 petunia plants that were given one litre water per day on average, produced a similar amount of flowers as baskets given 300 mls water per day on average with most of the excess wasted.
4. Very wet or very dry compost wasted the most water so try to keep things just moist. 
5. Allowing the surface of the compost to dry out isn’t a problem and can actually be an advantage in reducing water lost from the surface.
6. Containers that allow you to water below the surface of the compost (such as selfwatering baskets) keep the surface dry, reducing evaporation and have in built space to store water if it does rain.
7. Automated drip irrigation systems can stop wastage because the slow drips have time to soak in before the water runs away, and on a larger scale can be set up to match the rate that the plants are using the water.
8. A timed drip irrigation systems still needs adjustment to make sure it is delivering what the plant needs as it grows and as the weather changes. Soil moisture sensors are an improvement on this but still need checking to make sure that what they are measuring is representative of what the plants need.
9. Containers don’t need to be topped up full all the time and many plants will adapt by using less water if there is less available to them.
10. Save rainwater rather than using mains water because plants prefer it.  All the drip irrigation systems in the trial needed mains pressure to work apart from the solar powered pump system, which used water from a water butt.

Why self-watering baskets work:

Traditional hanging baskets are notoriously difficult to water and keep moist, but these baskets has a reservoir that means there is an extra portion of water that the plants can use on the hottest days.

Always fill the reservoir, rather than adding water to the compost surface so that the water and the wick stay clean. Watering from above will wash compost fines and nutrients into the reservoir and make it go smelly and anaerobic. Water into the reservoir and the plants will suck up water by capillary action and just take what they need.

Rather than overwatering, just because you can only water at certain times of the day, (which will encourage your plants to use more water), you can fill the reservoir anytime and know that you haven't saturated the compost.

By keeping the compost surface dry all the time, you also reduce evaporation so saving water as well. And there's more.... all that water running out the bottom of traditional hanging baskets means you are not only wasting water but leaching nutrients needed to keep plants flowering. There is little wasted with the reservoir to catch both water and nutrients when it rains. Water from below and these baskets work very well.