The city is planting a tree on the street near our pool. We have clay soil. The city has provided the following list of trees. Which one is the least messy?
Skyline honeylocust
Hackberry
Homestead elm
Greenspire linden
Crimson king Norway maple
London plane
Oak-leaf mountain-ash
Japanese zelkova
Katsura-tree
Thank you for providing an accurate descriptive list of the trees offered by the city. To determine which tree is least messy, we need to look at some of the growing conditions and the growing features of each tree. I have provided a brief comment on each tree in the list, with an authoritative source to read more details.
The Skyline honey locust, Gleditsia triancanthos f. inermis ‘skycole’ Skyline, an attractive tree, has one characteristic you may not appreciate. It produces 18″ seed pods containing a sweet, sticky substance (honey-like) which may be problematic for the filter system of your pool. https://www.chicagobotanic.org/plantinfo/tree_alternatives/skyline_honeylocust
Hackberry, Celtis occidentalis, is recommended by Landscape Ontario: https://landscapeontario.com/celtis-occidentalis-hackberry but the fruit berry attracts multitudes of birds, and the dropped berries do stain. If your pool is not close to the canopy and would not be vulnerable to dropped fruit, this tree may serve both your purpose and the city’s well. Although there are few problems with the hackberry, repeated postings on witches’ broom and chlorosis show up on the Internet. These are not serious but may detract from the attractiveness of the tree.
Homestead elm, Ulmus “Homestead’, is recommended on your list by the city because it is resistant to Dutch Elm disease, and may give you lasting pleasure as a shapely, elegant shade tree. https://landscapeplants.oregonstate.edu/plants/ulmus-homestead. My own experience with elm trees and a pool are the plentiful but tiny seeds. These seeds are no bigger than the tip of your little finger, are almost translucent, and can be problematic in the filter system of the pool. The seeds drop in great numbers in a few weeks in late spring. If there is a way to cover the pool on breezy days, you might overlook this issue for the benefit of the good qualities of the tree.
Greenspire linden, Tilia cordata Greenspire, sounds like your ideal tree from the following site, which mentions it is “tidy and low maintenance, …ideal near a pool” https://www.chicagobotanic.org/plantinfo/tree_alternatives/greenspire_linden
Norway maples, Acer platinoides Crimson king, have had a bad reputation as invasive for some time. The city of Toronto has considered not offering them to residents. The prolific production of seeds which easily germinate leads to crowding out of other vegetation, from smaller trees to groundcover plants. The following site gives cautious advice https://www.nanps.org/index.php/conservation/alien-invaders, while the Ontario Invasive Plant Council does not recommend their planting because of how great the invasive destruction of this tree has been and suggests alternatives to it. https://www.ontarioinvasiveplants.ca/wp-content/uploads/2020/04/Southern-Grow-Me-Instead-1.pdf
The London plane tree, Platarus x acerfolia, is admired by many gardeners and tree lovers for its shape, smooth grey bark, and ease of planting and early care. Do read about the problems that constitute the label of ‘high maintenance’ https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a892 but it is not a messy tree at all.
Next, the oak-leaf mountain ash, Sorbus x hybrida ‘Fastigiata‘, is an attractive columnar tree, but has two qualities you need to be aware of. It should be grown in full sun, and in soil with good drainage. The clay soil of your garden may be the same soil on the city land. Otherwise, the tree has many pleasing attributes and the small clusters of berries do not drop readily, but are eaten by birds directly from the tree.
Japanese zelkova, Zelkova serrata, is a very large tree demanding great open space for spread. https://canadiantreetours.org/species-pages/Japanese_zelkova.html#toronto. It may not be messy, but its demands are greater than some of the other trees we are examining here. https://landscapeplants.oregonstate.edu/plants/zelkova-serrata
The katsura tree, Cercidiphyllum japonicum, is described in the following site as perhaps ideal for city streetscapes. https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=j710 Note that as with most trees there may be dropping of the small, insignificant blossoms in spring, or by the little pods or seed off the tree. The foliage is attractive but doesn’t seem to cause much littler.
You have good choices to make, and the information here may help you to narrow the list more. Best wishes for a beautiful tree.