The science behind plants and gardening
Gardeners’ Lore
1. Sow seed with a waxing, never a waning moon
Possible Explanation: We don’t think that this has any real basis in fact, despite being promoted since ancient times. Some hold that the moon affects the earth’s atmosphere so that statistically it is more likely to rain heavily immediately after a full or a new moon. We don’t think this is so and heavy rain can also inhibit seed emergence.
Others suggest that the effect of lunar rhythms on the earth’s magnetic field affect growth. Or that the ‘tidal’ forces generated by the moon affect soil moisture supply. Again there seems little evidence that these actually have an effect on seedling emergence or on plant growth.
If there is an effect it is likely to be much less significant than water supply from rain or irrigation or than soil or air temperature.
2. Incorporate scrapings of topsoil from below pines and spruces when planting, and then mulch with pine and spruce needles, crushed fir cones and even twigs to improve the strawberries’ flavour
Possible Explanation: Strawberries are a woodland plant and love organic matter and mild acidity. At one time they were a preferred crop to grow on cleared woodland. Material from beneath conifers is rich in organic matter and often quite acid. As stripping topsoil from woods is frowned upon and as few gardeners have many fir and pine trees, composted bark should make a good soil improver before a strawberry crop. However, at a pinch any well-rotted organic matter will be helpful.
Even so, adequate supplies of nutrients are important and these can be easily supplied by a modest application of general fertiliser.
3. Plant marigolds in the flower and vegetable garden and greenhouse
Possible Explanation: These plants are considered to generate materials that have an effect on soil-dwelling nematodes and against aerial pests on nearby plants. They are known to have some effect on certain nematodes, called root knot nematodes which are not a significant pest in the UK. In Britain they are widely considered to protect plants against whitefly. We are unaware of any good evidence that this really is so, and think that when plants remain free of whitefly in the presence of marigolds, it is highly likely that they would be whitefly-free even if there were no marigolds nearby.
In the past at Wisley we have had marigolds supporting thriving colonies of whitefly. Give them a go and let us know what you think. However, introducing the very effective whitefly parasites, planting nematode-resistant plants, and also crop rotation to control nematodes are more reliable.
4. Grow foxgloves in your garden to stimulate growth and disease resistance among your other plants
Possible Explanation: Although there seems to be no evidence of any material given off by foxgloves or an effect on nearby plants, there may be a basis in fact for this.
Mixed communities of plants often seem to be less afflicted with pets and diseases. Therefore gardeners who allow foxgloves and other plants to distribute themselves through the garden will encounter fewer severe pest and disease problems, simply because diseases and pests have more difficulty in finding hosts in mixed communities. Remember that pests and diseases are often quite specific about which host that they will attack and only go for these plants.
Another reason may be that foxgloves have deep roots that extract nutrients form deep in the soil and their fallen leaves nourish less deeply-rooted plants. In well-manured and fertilised garden soils this seems unlikely.
5. A single clove of garlic planted beside a rose will keep greenfly away from the plant
Possible Explanation: Although there seems little real evidence that the pong of garlic plants will reliably deter insects from roses, there is no reason not to try this, as even if it does not work you will get a nice bunch of garlic at the end of the season.
Using an approved insecticide to knock out individual colonies of greenfly, rather than overall spraying of plants, is a better remedy. There are some effective plant-based insecticides for use by people who are uneasy about using synthetic chemicals.
Of course, with luck natural predators including birds will eliminate most greenfly on roses.
6. Bananas planted under roses will ensure strong, healthy plants
Possible Explanation: Proponents suggest that high quantities of calcium, magnesium, sulphur, phosphates, sodium and silica are released into the soil. In fact, the nutrient levels are very low in comparison with manures and fertilisers and the effect of bananas as fertilisers on well cared for soils is likely to be insignificant. However, in ill-prepared soils, even the tiny levels of nutrients in a banana may have a useful effect. Much better to put banana skins - and any other material that you can lay your hands on - on the compost heap. Compost is the best fertiliser a gardener can use.
However in ill-prepared soils, even the tiny levels of nutrients in a banana may have a useful effect.
7. Pour left over tea into your houseplants
Possible Explanation: Tea is likely to have the properties of a mild fertiliser. Dissolved matter should provide some nitrogen and is also likely to be mildly acidic from the tannins found in tea. It is also made from boiled water, so this will also reduce its alkalinity or hardness. Acid-loving plants such as azalea and camellia are reported to benefit from this watering with tea. Even so, best growth and flowering is likely to come from using fertile potting compost and fertilisers specially formulated for acid-loving plants.
8. The liquid run-off from compost heaps, known as ‘compost tea’ helps keep plants healthy
Possible Explanation: Run-off from compost heaps has some nutrient content and it is a useful liquid fertiliser when dissolved to the colour of weak tea. Even so, its nutrient content will be very low and ailing plants will benefit from stronger, purpose-made proprietary liquid fertilisers.
Interestingly, certain kinds of compost tea have been found to have properties that appear to protect plants from diseases, when applied to foliage and roots. At the moment this is still under investigation and we cannot make firm recommendations. This is an example of where the RHS is following research to interpret it on behalf of gardeners.
Why are plants green?
They contain the pigment chlorophyll, which absorbs blue light, and a certain amount of red light, to provide the energy for photosynthesis. Green light is reflected so that plants appear green to us.
Why do some flowers smell and others don’t?
Flower scent, along with flower colour, is a means of attracting insects to visit the plant where they can collect a ‘reward’ of sugary nectar and in doing so will pick up and deliver pollen.
Scents are often especially intense in the evening when moths are on the wing and low light levels make it difficult for animals to perceive flower colour. Tobacco flowers, honeysuckle and lilies are examples of these.
Some plants use pollinators that find flowers by sight alone, such as humming birds. Bright colours rather than scent attract these and therefore no scent is required. Making scent demands a lot of energy from plants, so if at all possible they avoid producing it.
Wind pollinated plants, of course, have no need for scented flower.
Why do plants grow to towards the sun?
Life is a struggle for plants. They must capture as much light as possible before neighbouring plants shade them out. Hence they have evolved the means to grow towards light. Curiously, some seedlings of some tropical climbers, such as Swiss cheese plant (Monstera deliciosa) are said to climb towards darkness, no doubt because the base of tropical trees are dark and that is where the climbers must begin their ascent.
Want to know more?
The following RHS publications can help:
Learn to Garden: Perfect for the beginner gardener as it demystifies the science behind gardening so you get better results first time.
Science and the Garden: An excellent book for keen gardeners and those studying horticulture. It doesn’t just tell you when do things, but why.
For information on RHS publications click here
The RHS employs around 20 scientists, most of whom are based at RHS Garden Wisley. They endeavour to give gardeners a practical understanding of the science that underpins gardening, to improve their experience of horticulture. They develop ideas on best practice in gardening, which meet the needs and expectations of gardeners and respect the fragility of our environment. The research they undertake yields practical benefits for gardeners at all levels of experience and interest. These are the people who can sort the fact from the fiction!
"Plant science is about asking questions and testing theories to see why and how plants grow as they do," said Dr Chris Prior Head of Horticultural Sciences. He continued, “This is something that gardeners do in their own way - in order to learn from the experiences in their gardens.”
From plant pathology and soil science to identifying rare plants and new species, horticultural science is a fascinating subject.