Ageratum houstonianum – a plant with a secret defence mechanism

We have grown Ageratum houstonianum, the Mexican Paintbrush, for a number of years now. Originally we knew Ageratum as one of those short carpet bedding plants that you would see alongside red salvias and orange marigolds in council bedding schemes in the local park.

Some ten years ago we started up Honey Pot Flowers and were trying out new varieties to grow and include in our summer bouquets of British country flowers. We were introduced to the F1 variety ‘Blue Horizon’ which produces flowers on much longer, robust stems and holds extremely well as a cut flower. We have been growing it ever since.

This variety of Ageratum has large, dense flower heads of powder blue. It is not a colour that you find very often in the garden and so makes an interesting addition to both the flower borders and cut flower arrangements. Most importantly, once established it does seem to be a very well behaved plant, growing and flowering reliably throughout the summer.

It is a native of Central America and Mexico and it is worth noting that in some countries it is considered an invasive weed. We have certainly not found this to be a problem here in the UK climate.

I have just sown this year’s seeds (14th February). The tiny seeds are surface sown on a half tray of pre-watered, well drained multi-purpose compost. To ensure the seedlings do not damp off in their early days we mix the compost with a generous amount of perlite. I like to water the compost before sowing so that the tiny seeds do not get washed into the corner.

The half tray is covered with cling film to remove the need for any extra watering before the seedlings emerge. They will sit on a warmish kitchen windowsill now for a few weeks until the green shoots appear at which point we will slowly loosen the cover and acclimatise the small plants before removing the cling film altogether.

Looking at previous years’ records I will be pricking these out into larger trays in about five weeks time (towards the end of March), and will have planted them out into the garden by 6th May. They are not frost hardy so this would need to be adjusted to suit your own circumstances.

Our past records also indicate that we have been picking Ageratum for cutting by mid-June and they then continue to flower right through to the first frosts in the Autumn.

Ageratum ‘Blue Horizon’ arranged together with Phlox, Aster x frikartii ‘Monch’, Liatris and pink Clary Sage

Over the years we have tried them in various parts of the garden but they do seem to grow best in full sun in soil that remains moist all the summer. Our garden soil can dry out rapidly in the height of the summer but there are areas in part shade that retain some moisture in the soil. The Ageratum also seem to thrive in these areas.

What we particularly like about Ageratum is how trouble free it is. It does not seem to be attacked by pests which is ideal if you want to grow good quality, clean stems for cutting, arranging and/or selling. In researching the plant this year I was surprised to find that it has its own secret defence mechanisms when it comes to fending off insects.

Ageratum has evolved a method of protecting itself from insects by producing a compound that interferes with the insect organ responsible for secreting juvenile hormone during growth and development. The chemical triggers the next moulting cycle prematurely and renders most insects sterile. Fascinating stuff!

Small blue and white arrangement combining Ageratum ‘Blue Horizon’, chincherinchee and scabious seed heads
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Zinnia elegans – a hot climate plant from Mexico loving the 2018 UK heatwave

One flowering annual that has thoroughly enjoyed the hot weather this year has been the Zinna.  Whereas the Dahlias appear to have been delayed this year the Zinnias have produced tall, strong, clean plants with masses of excellent quality flowers.

Zinnia Lilliput Purple P1010994
Zinnia elegans ‘Lilliput Purple’

I must admit that in previous years my Zinnas have struggled and if you get a cool period they just seem to sulk.  In order to try and improve their chances in the future, even in cooler years, here are a few things to consider.

  • Grown from seed Zinnia will germinate in 3-5 days at 27-29°C and 5-7 days at 21-24°C.¹
  • The seedlings should be grown on with night time temperatures of 15-18°C and day time temperatures of 21°C.¹
  • Zinnas dislike root disturbance so ideally the seeds should be sown in individual modules to reduce disturbance at the planting out stage.
  • Plant out at a spacing of around 6 inches apart.  The denser the spacing the taller the plants.¹
  • Zinnas are quantitative short day plants (see How plants use day length to decide when to flower (Photoperiodism) ).  This means that they flower more rapidly under short days but eventually flower regardless of photoperiod.   Long days (greater than 12 hours) produce longer stems but delay flowering.  Similarly if you plant a succession of Zinnas after the longest day they will flower on shorter stems.¹
  • Benzakein⁴ emphasises the importance of pinching out the central flower bud when the plants are 18 inches tall to encourage the development of lower branches on the plant.
  • Zinnas do seem to be prone to powdery mildew particularly if growing in conditions where hot days are followed by cool damp nights.  Over watering can cause similar problems.
  • If you are growing for cutting, pick the flowers before the pollen matures.  If you can see fluffy, mature pollen the chances are the bees have done their job and the flower is now redundant and will wither to allow the seed to develop.
  • There seems to be a difference in view as to whether Zinnas like ‘flower food’ or not.  Beutler² states very clearly that they do not like floral preservative whilst Benzakein⁴ is an advocate of its use with Zinnas.
  •  Zinnas are long lasting in water but will wilt rapidly out of water and it is reported that they do not work well in corsages and buttonholes.²

In reality there is little that many of us can do about the great British weather and in some years different species will grow better than others.  Certainly Zinnas like it hot and starting to grow the seeds too early if you don’t have the facilities to keep them warm can reduce your chances of success.

Benary's Giant Wine
Zinna elegans ‘Benary’s Giant Wine’

Keep in mind that Zinnia elegans is a warm-hot climate plant native to Mexico growing in scrub and dry grassland.  Not every year will be suitable for them here in the UK but their bright vivid colours in late summer are certainly striking when conditions are right.

Half Hardy Annual

Origin:  Mexico

Height:  60-90cm

Flowering period:  July to October

Latin name:  Zinnia elegans (after botanist Johann Gottfried Zinn (1727-1759)) ¹

Family:  Asteraceae   (Tribe:  Heliantheae – Sunflowers)

Further reading

¹  “Specialty Cut Flowers” by Armitage and Laushman (ISBN:  0-88192-579-9)

²  “Garden to Vase” by Linda Beutler (ISBN: 978-0-88192-825-9)

³  “The Cutting Garden” by Sarah Raven (ISBN: 978-0-7112-3465-9)

⁴  “Cut Flower Garden” by Erin Benzakein and Julie Chai (ISBN: 978-1-4521-4576-1)