I’m pleased to write that I have finally managed to track down some sweet crabapple (Malus coronaria) fruit. Sweet crabapple is native to the Northeastern United States and Canada, and is one of four apple species endemic to North America. Sweet crabapple is not named so because of its fruits, which are actually rather sour and bitter, but the flowers, which are reported to smell like roses.
I have been interested in the native crab apples of North America for a few years. Pacific crab apple (Malus fusca) is of most interest to me because of its tolerance of wet soils, being able to grow in standing and brackish water (!). These traits could be very useful for breeding apple rootstocks that don’t just tolerate wet soil, but thrive in it. I believe some unidentified crab apples in New Zealand are actually Pacific crab apple, so I believe it is here although currently unrecorded.
Sweet crabapple was not a species I was originally interested in. However I came across an article mentioning how apple breeders in the USA are looking to use its later flowering ability to breed new apples that avoid late Spring frosts: https://www.michiganpublic.org/environment-climate-change/2025-01-20/a-key-to-protecting-apples-from-climate-change-might-be-hiding-in-michigans-forests
The article describes how these native crab apples are beginning to become very rare, and so they are in a hurry to preserve them before they are lost. Sweet crabapple blooms about 2-3 weeks later in Michigan. They estimate it could take decades before these genes appear in commercial apple varieties.
One of the more severe effects of climate changes is that the weather will become increasingly erratic, such as late Spring frosts becoming more common. We don’t have much of a late frost problem on the West Coast here, but we have a related problem which is spring rain and wind that blow all the flowers off or prevent effective pollination. Usually it’s plums that suffer the worst of it, but apples can sometimes have poor crops too. So the ability to spread the flowering period of apples 2-3 weeks later could be very useful for food security.
Secondly, sweet crabapple fruits can last a very long time outdoors without becoming rotten. As 18th century description from botanist Pehr Kalm noted:
“They lie under the trees all winter and acquire a yellow color. They seldom begin to rot before spring comes on”
This is the aspect that was most interesting to me, as I’m very interested in “super late apples” from the food security aspect. I suspect most of the super late apples in New Zealand come from a very narrow pool of genetics, and so it would be difficult to breed more out of them without additional genetics. By having a completely separate apple species that has its own set of “super late” genes, there is a lot more scope for breeding other apples that can last so late.
Lastly it seems sweet crabapple is sometimes used as rootstock, where it is known for its hardiness (although I’m not yet sure exactly what this means).
So here I was at an arboretum in the North island, scrounging around in the underbrush with a staff member looking for a tree that probably doesn’t get more than a few visits per decade. The trees there were all tall, with canopies far out of reach and hardly within sight. Soon however I spied some shining little green balls lying on the leaf litter, and I knew instantly that we’d found it.
The fruits were small, being roughly the size of a cherry. They have a sticky, waxy skin on them, like a really past-it old supermarket apple does. This was a bit surprising to me. I suspect this thick waxy layer is a key part of why the fruits can last so long.
The fruits themselves are strongly fragrant, with a scent similar to common apples but with much more of a floral character. I’ve probably smelled the same chemical before in certain soaps, but I tend not to eat soap so I can’t be too sure.
The fruits when chopped brown relatively quickly, which I found surprising. As for taste, they have a relatively clean tasting sourness in the initial taste that varies in intensity from as sour as a lemon to much less sour, based on the age of the fruit. It seems like there is maybe only malic acid in these fruits, and other Older fruits are noticeably less sour. It will be interesting to taste them in a few weeks. Surprisingly the taste is not very reminiscent of green apples – these have a much cleaner acidic taste. There is a light sweetness to the fruits, but it’s hardly noticeable compared to the sourness. Many fruits were lightly bitter as well, with the most bitter being perhaps up to half as much as a grapefruit. The floral scent is also experienced as a flavour that often blends into the bitter taste. I found myself unexpectedly enjoying eating little slices of these.
Sweet crabapple is actually not too uncommon in New Zealand, but is found almost exclusively as the ‘Charlottae’ cultivar grown as a flowering ornamental, which has a double shell of flowers. I wasn’t sure if the ‘Charlottae’ cultivar is actually representative of wild Malus coronaria or not. Is it a strange mutant, a pentaploid or maybe it’s a hybrid? Hybrid crab apples are often apomictic (= mother tree clones itself via seed) tetraploids, and so then they are not as useful for breeding.
As it turns out, practically all populations of Malus coronaria are apparently tetraploids, with very high rates of apomixis as well (about 75% or so), and so it seems many of the seedlings I’ll end up with will be clones of the mother tree. This is a double-edged sword. On the one hand, I absolutely could not reach up to get scionwood of the mother tree as I think the canopy was only starting at about 6 metres or so, so apomictic seedlings means we’ll have the exact mother plant represented in seedlings rather than seedlings hybridized by random crab apples. Similarly by identifying a high proportion of seedlings as being apparently identical, we can be sure that the parent tree was Malus coronaria and not a related species such as Malus ioensis or Malus angustifolia (both are primarily diploid).
On the other hand, it means breeding with them will be difficult. Most seedlings will be apomictic clones, and the F1 hybrids would mostly be triploids, and then their offspring will be also mostly triploids and tetraploids. Getting back to a diploid is probably better for incorporating into a normal apple breeding program, but I’m not yet sure how to detect these among seedlings
For now the next steps are to try and sprout these seeds, and to see how long the rest of the fruit last, and what they taste like after as much storage as possible.
References
- https://www.michigannativeapples.org/about
- https://www.michiganpublic.org/environment-climate-change/2025-01-20/a-key-to-protecting-apples-from-climate-change-might-be-hiding-in-michigans-forests
- Keeler, Harriet L. (1900). Our Native Trees and How to Identify Them. New York: Charles Scribner’s Sons. pp. 133–5
- https://blogs.cornell.edu/master-gardeners-cce-oc/2021/06/30/tree-of-the-month-sweet-crabapple-tree/
- Kron, Paul; Husband, Brian C. . (2009). Hybridization and the reproductive pathways mediating gene flow between native Malus coronaria and domestic apple, M. domestica. Botany, 87(9), 864–874. doi:10.1139/b09-045Â
- https://fedcoseeds.com/trees/charlottae-crabapple-7306