Senna cardiosperma – Fabaceae

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Senna cardiosperma – Fabaceae

Because of the considerable variation of Senna spp., plus their tendency to readily hybridise, these plants are often very tricky to identify as many past classification attempts can testify. However, Senna cardiosperma is one of the more obvious due to their compressed foliage arrangement. The leaflets can number between 2 and 14 pairs, although around Esperance they are usually 3 or 4 pairs. The important feature is the leaflets are less than 5 mm apart and less than 15 mm (5/8”) in length. Consequently they are tightly spaced along the rachis (stalk), but if the spacings or leaflet size is greater (locally not uncommon), they are either a different species or a hybrid.

Senna cardiosperma is an inland species and common in the Kalgoorlie region. Around Esperance it occurs in the mallee, arcing 100 km (over 60 miles) NW to the NE of the town and becoming more common the further north you go. Like other Senna spp. it germinates readily in a variety of well-drained soils after bushfire or soil disturbance and can grow to around 2 metres (6’) in height, although plants are usually shorter and quite bushy.

The flowers tend to be distinctly yellow when growing in a sandy loam over limestone, but develop an orange tinge when over gravel. Senna cardiosperma is a prolific producer of flowers that are shown to advantage by easily exceeding the squat compressed foliage near the stem. Flowering times locally are usually between July and November, but can also bloom later if local rainfall is drenching and temperatures mild.

Senna pleurocarpa - varieties pleurocarpa and angustifolia

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Senna pleurocarpa

Senna pleurocarpa has three varieties of which two occur in Western Australia and both are found in the Esperance region (third variety restricted to Qld).  The variety angustifolia is restricted to WA, whilst var. pleurocarpa occurs in all mainland states except Victoria.  The local varieties mainly differ from each other by the number of leaflets each has, with var. pleurocarpa having 5-7 pairs and var. angustifolia 8-10.  Mention is also made of the bracts, leaflet and pod size, although locally there is very little difference between them and sizes sometimes are greater or smaller than those specified.

The number of leaflets I find also varies, with var. pleurocarpa ranging from 3-7 pairs and var. angustifolia from 7-9 pairs even on the same plant, so locally identification can be tricky and why I include both in this post.  It is highly likely that other forms exist elsewhere that differ more, but they do come close to merging around Esperance.  Both varieties are fast growing, but relatively short-lived, so need regular bushfires or soil disturbance to keep them around, otherwise they disappear until after the next bushfire.

Senna pleurocarpa var. pleurocarpa

This variety I encounter more on and around granitite outcrops, with seed being distributed (probably by birds) into surrounding areas to produce plants in differing habitats, but usually on disturbed soils.  It commonly grows to 1.5 metres (5’) in height, although it can grow taller and is recorded flowering from July to October.  However locally September to November is more usual.   Most of my sightings of this variety have been to the north and northeast of Esperance where granite outcrops are more common.

Senna pleurocarpa var. angustifolia

This variety I find more often in sandy loams, particularly over limestone and mainly to the NW of Esperance, but also in the NE to overlap with var. pleurocarpa, although not growing together.  It tends to spread more and is commonly less than 1.5 metres (5’) in height.  Flowering is recorded anytime between July to April, although local rainfall would have a considerable influence and in summer dominant rainfall zones (where this plant also grows), so would probably bloom then.  Around Esperance there is little difference in the flowering period of either variety, probably reflecting similar weather conditions.

Senna sp. ‘Pallinup River’ – Undescribed S. artemisioides subsp.

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Senna sp. ‘Pallinup River’ – Undescribed S. artemisioides subsp.

Senna sp. ‘Pallinup River’ has been lumped in with the variable and general grouping called Senna artemisioides subsp. x coriacea, but they have distinctive boat shaped leaflets that are longer and thinner, with plants ranging from the Nullarbor to Albany and extending up to 300 km (185 miles) inland. The still variable Senna artemisioides subsp. x coriacea is recognised as a hybrid/s and also occurs in the Esperance region, but locally is rarely encountered.

Like many Senna species, this one has a liking for limestone and in this instance favouring a non-calcareous overlay of fine sand/clay. It also has a higher moisture requirement than most other WA sennas and is commonly found around roadside culverts and granite outcrops, where rainfall run-off is channeled and soils remain moist for longer periods. They also do particularly well after bushfires or where the soil has been disturbed, although their longevity is dependent on the growth of taller shrubs that will eventually dominate sunlight and moisture availability, consequently they are often short lived in these situations.

Senna sp. ‘Pallinup River’ grows to a metre (3’) in height and can be reasonably bushy with pleasant pale green divided foliage (usually in two pairs). The bright golden yellow flowers (little over 1 cm or ½” diameter) are quite prolific and providing the environment retains moisture, will flower over several months by producing additional blossom on any new growth. The most intense flowering period is from August to November, but depending on local rainfall, flowers can also be produced at other times.

Senna artemisioides subsp. filifolia – Desert Cassia

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Senna artemisioides subsp. filifolia – Desert Cassia

The Senna genus has recently been moved to the Fabaceae family, earlier having been placed in Leguminosae or Caesalpiniaceae. Previously the genus was also known as Cassia, a genus that is no longer regarded as occurring naturally in Australia. The species name has also changed from Cassia nemophila and Cassia eremophila to Senna artemisioides. If that were not enough, Senna artemisioides subsp. filifolia is also known as Punty Bush or Fine-leaf Desert Cassia. So as you will appreciate this group of plants can be tricky to identify, as combinations of the above are listed in various botanical literature.

Thankfully, this species with its fine 1 to 4 paired leaflets (mostly a single pair locally) is easy to identify in the Esperance region, where it is quite widespread and largely confined to mallee country (with a limestone substrate) in the north and northeast. However, it also occurs in inland regions of all the mainland States and in WA is not much different, avoiding the cooler/wetter southwest and the west coast, although being common further inland. It is highly probable that this species is spread even further afield by vehicle traffic, as it is a colonising species after bushfires and disturbed soils along roadsides.

Despite its inland preference, it seeks moisture by growing in (non-flooding) moist hollows and around rocky outcrops (usually with a limestone topping); it is also found in flat sandy regions. Amongst tall mallee vegetation it can grow to 3 metres (10’), but in most instances is around half that height. The yellow buttercup-like flowers are about 1 cm (nearly 1/2”) diameter and quite prolific, with new ones developing as the plant grows. However, being so widespread, plants are subject to a considerable range of climatic conditions and varying rainfall, therefore will bloom depending on local conditions, which tend to be after rainfall at the cooler time of the year, so unless it is hot and dry, flowers can invariably be found. Locally, August to October would be the best flowering period.

The pods are initially green and form in considerable quantity, but as they mature turn to a grey/brown that burst open when sufficiently dry to scatter the seed. The flowers are pollinated by various insects and are also visited by butterflies and small birds.