Amazing annuals: Isotoma

Delicate, fragrant and unusual, this neglected annual is lovely, whatever its name.

There is a lot to like about Isotoma axillaris (often called Laurentia or Solenopsis), a tender perennial from Australia usually grown from seed and treated as an annual. It is just what we want in a summer flower and plant for summer pots. It has attractively toothed, deep green leaves, forms bushy plants about 25cm high and wide and once it starts to bloom it just gets better until late autumn. The flowers are delicate and starry, about 3cm across, held above the foliage, and lightly fragrant. They are usually pale, (lavender) blue but can be white and or pink.

Isotoma is in the Campanulaceae and closely allied to lobelia. The flowers do indeed resemble lobelia or pratia and, for me, perfectly combine a good splash of colour with interesting flower form and compact, but not stunted, habit. They are not the easiest plants to grow from seed but not difficult either – they just need a bit of extra care. The seeds are small and although I am not sure, I suspect that they need light to germinate so do not cover them with compost but use perlite. Make sure that they are kept moist but avoid soggy compost at all times. Once the seedlngs produce true leaves they can be transplanted into cell trays. They have a few, relatively fleshy, white roots and need careful handling but once they settle and start growing they do quite well. Wet is the biggest problem, especially if it is cool. The trouble with cell trays is that it can be difficult to water each plant individually. If one plant grows well and fills the cell with roots it will dry out more quickly and that means slopping water all over the other seedlings which may already be wet. The excess water pushes these wet plants back further. It may be better to grow them in ordinary trays so the watering is more even. Once the seedlings are all growing well they can be planted out in late spring or moved to 8cm pots to grow on.

In addition to the species, Thompson & Morgan bred the F1 ‘Avant-Garde’ in blue and pink and they gained a Fleuroselect Gold medal in 2007. These are supposed to be earlier to bloom and have better branching. I have grown them, but not side by side with others, and they are great.

I find that Isotoma grows well in ordinary sunny borders and very well in patio pots. When the flowers fade they are replaced by long seed pods and deadheading would keep plants blooming better. Even so the plants are relatively self-cleaning, the new growth engulfing the old.

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One Comment on “Amazing annuals: Isotoma”

  1. tonytomeo
    January 1, 2021 at 7:17 am #

    I think this is used as a perennial ground cover. It was popular years ago, but I have not seen it in many years.

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