If unpaved drainage ditches and collection ponds were more common locally, sweet flag, Acorus gramineus, might be also. It can provide a nicely neat border for such waterways, where the ground is too steep and damp for mowing. It can migrate into muddy situations and even into shallow water. Its dense network of fibrosus rhizomes helps to retain mud.
‘Variegatus’, with elegantly elongated and variegated leaves, is the most popular cultivar locally. It is rare in nurseries, but occasionally shared by friends and neighbors who grow it. Propagation by division is very easy. Single shoots or clumps of shoots grow if merely plugged into damp soil or mud. ‘Pusillus’ lacks variegation, and develops stouter leaves.
Sweet flag aggressively excludes other herbaceous vegetation, but does not migrate too rapidly. Plucking shoots and plugging them elsewhere accelerates migration. The dense foliage might get a foot deep. Individual leaves are very narrow, like grass. The mundane bloom is easy to ignore and is uncommon where the soil is not often saturated or muddy.
Ducks somehow find water. They eventually visit most home garden ponds that they can fit into. Duckweed, Lemna minor, is likely to come with them. It adheres to waterfowl and other wildlife for that purpose. It proliferates very efficiently, and almost typically becomes a nuisance. Eventually, proliferation in a healthy pond should stabilize to a tolerable rate.
Individual duckweed plants are tiny. Their oval leaves are typically less than a quarter of an inch long. Each floating leaf extends its single root less than three quarters of an inch into the water below. Plants produce no more than four rooted leaves before dividing into a few smaller plants to repeat the process. Bloom and subsequent seed are uncommon.
As a floating aquatic plant, prolific duckweed might obscure koi and submergent aquatic plants within garden ponds. However, it also helps stabilize healthy aquatic ecosystems. In fact, it is useful for bioremediation of agricultural and industrial applications. It absorbs detrimental substances from water, while producing fodder and biomass for composting.
Home gardens and landscapes should be compatible with their respective climates. For local chaparral climates, plants that do not need much watering through the long and dry summers are appropriate. Aquatic plants are the extreme opposite. They require regular replenishment of the ponds that they inhabit. Arid warmth increases their need for water.
Aquatic plant can not be ‘drought tolerant’. Several, such as duckweed, water lettuce and water hyacinth, float over the surface of water. Water lily and lotus inhabit the mud below the water, and extend their foliage to float over the surface of the water. Waterweeds stay completely submerged, with or without roots. Aquatic plants need water for their survival.
Marginally aquatic plants are somewhat less dependent on water. Cattail and yellow flag iris inhabit shallow ponds and saturated soil, but can survive if their situations drain for a while. If they stay too dry for too long, they can initiate dormancy, and then recover when saturation resumes. Canna inhabits either shores of shallow ponds, or evenly moist soil.
Regardless, all aquatic plants require maintenance that is completely different from what terrestrial plants require. Even those that need only minimal maintenance will eventually necessitate muddy and messy interaction. Much of such interaction is under water that is difficult to see through while murky. Aquatic plants are innately heavy and totally sloppy.
Moreover, some common aquatic plants grow like weeds. Water hyacinth and giant reed are two of the most invasive exotic species in California. So, not only are they sloppy, but they are also voluminous! Because they are very invasive, they should not inhabit ponds that they could escape. Besides, giant reed is too overwhelming for most home gardens.
Few home gardens include natural ponds or water sources to contain as ponds. Garden ponds are therefore mostly contained within some sort of sealed infrastructure, and need replenishment to compensate for evaporation. Fountains aerate the water for a healthier ecosystem, but also increase evaporation. Some tall aquatic plants also consume water, as the foliage that extends above water transpires.