Hello Sara,

Yes, wild trees certainly do grow virtually on top of each other, relatively speaking, and they do compete for growing space (both above and below-ground), moisture, and nutrients. Some botany researchers are studying whether or not it is actually better for tree health for them to grow in closer proximity (closer than we tend to plant them horticulturally, that is) so their fungal root network interacts and (potentially) shares resources and signaling chemicals that help other individuals avoid insect attacks. It is thought that related (either the same species or literal "mother"/"daughter") trees may benefit from this but unrelated trees (like different species) may not - in the latter case, the competition outweighs any possible benefit. More research is definitely needed to understand this very complex relationship and the interconnectedness of a forest or wooded landscape.

In horticulture, planting too close can result in aesthetic decline over time, notably among evergreen conifers who do not replace foliage that is shed prematurely on parts of the plant that are experiencing crowding. Deciduous plants and some broadleaf evergreens can lose surplus foliage on heavily-shaded branches as well, but they have more potential for growing it back should conditions improve (such as if that close neighbor were moved elsewhere). Competition between crowded plants could slow growth and stunt the plant's mature size, or create asymmetrical or visually unappealing branching as they try to grow around any obstacles to obtain enough light. A more "wild and woolly" look is certainly fine and may even look more natural over time, so this in and of itself isn't necessarily a major detriment to the trees' health. Since gardening aesthetic usually desires plants of more even shape and symmetry, this is one reason overcrowding is discouraged in horticulture.

Southern Magnolia variety 'Little Gem' is a dwarf with a slower growth rate than the wild type. American Holly is quite slow-growing by nature, especially considering its eventual size (other hollies that mature large have faster growth rates). In ideal growing conditions, the holly would eventually mature about twice the size of the 'Little Gem', though this will still take decades either way. Magnolias have relatively shallow root systems and hollies react poorly to being transplanted at the wrong time of year (they often respond by shedding nearly all of their leaves), so moving either years down the road as they become too close to each other will likely not be easy, especially since their roots will become more intertwined over time. It would be far better to give them the spacing they will eventually need sooner so each has a better opportunity to thrive. This does not have to mean that the plants get the full range of their eventual width to themselves, but ideally much more than the current 3' plan. 'Little Gem' matures approximately 10' wide; American Holly up to around 15' wide. A better trunk-to-trunk spacing between any two trees would be at least 6', though even this is fairly close long-term.

If you have the space, one way to circumvent some of the recommended distance while still having the plants look fairly closely spaced is to stagger them in two different rows. Essentially, plant one row (of, say, the holly) at 8' spacing and, in front of them (by the same distance, about 8' in this example), plant the magnolias 8' apart, but in such a way that each magnolia is between two hollies. (Zig-zag fashion.) From a viewpoint at the "front" of the planting you wish to see this screen, it will look like the trees are only 4' apart, but they will still have enough room to mature into fairly even, dense canopies that serve as an effective screen.

If the trees haven't been planted for too long (maybe a couple of years or less, though this isn't a precise timeframe), then you can still transplant them if you wish to space them out further. This late in the season, it would be a better safeguard against winter damage to wait until spring to dig and move them, since they're evergreen and more vulnerable to winter desiccation otherwise. For the holly, some local in-ground nursery growers move theirs in late spring, or else they experience drastic transplant shock and take longer to recover.

If left in place instead, all you can do to minimize competition and promote their best growth is to water well when needed (such as in dry spells in summer and autumn) and mulch the root zone to discourage weeds.

Miri