This post should have been in H really because the proper name for it is Heteroblasty but many garden plants are known for their juvenile growth or foliage. Well I say many but two are obvious and frequently grown in gardens: ivy and eucalyptus. Ivy is the most frequent so I will start with this native plant. It has two very distinct growth patterns with very different leaves. In its juvenile form ivy (Hedera helix) is a rambling plant that creeps along the soil. It has leaves that are deep green and usually three-lobed. Ivy is a very variable plant and there are countless variations in leaf shape and lots of variegated forms. Ivy is searching for light. The flowers are pollinated by insects and there is no point flowering in the shade on the woodland floor. So ivy finds something to climb up and when it gets into the light, at the top of its support, the plant changes entirely.
Instead of wiry stems the growth becomes more rigid and upright, the leaves become simpler in shape and the end of every shoot carries a cluster of flowers, arranged in spheres. Quite how the plant knows how to do this is a bit of a mystery. If you take cuttings of the juvenile form they remain so and the same goes for the adult, fertile growth too. These cuttings of the adult form make attractive, shade tolerant shrubs although, in my experience, they sometimes produce some juvenile shoots too.
A more extreme example, though no more remarkable, is seen in the New Zealand native, Pseudopanax ferox, also in the ivy family (Araliaceae). In its early stage the single upright stem has long, linear leaves, tough and edged in spines almost like a saw and stiffly down-pointed so that nothing could climb it. Then, when it gets to about 2m high the growth changes as it branches and the leaves become digitate, like the related scheffleras. It is thought, and it makes sense, that this is to prevent being grazed by Moas, there being no other tall herbivore in New Zealand. Many acacias have a similar habit, producing pinnate leaves at first and then simple foliage.
But back to gardens, there are eucalyptus. In their juvenile forms they have thin-textured, blue-grey leaves in pairs up the stems and the growth is distinctly upright. At this stage they never bloom. After a few years the growth changes and the leaves become sickle-shaped, hang on long petioles from the stems and are usually greener. If the plants are cut back into the juvenile growth, more juvenile growth is produced and that is what we tend to do with E. gunnii, to keep up production of attractive, grey juvenile growth.
From personal observations I have seen the same thing in Pinus canariensis, a tree that grows on most of the Canary Islands. In the juvenile stage the leaves are short, singly arranged and greyish in colour. As the seedlings get older the leaves become much longer and in clusters of three. Once in the adult stage juvenile foliage is rarely produced unless the tree is damaged by fire but I have observed juvenile foliage growing from the bark of seemingly healthy trees. I am not sure why there is this difference but it could be that the grey foliage of the young plants resembles that of grey-leaved artemisias and other scrubby plants that may be unpalatable.
As we approach Christmas, it reminds me that you often see small ‘Christmas trees’ in pots for sale, about 20cm high, covered in fake snow. I am sure these are Pinus canariensis seedlings though I believe that Pinus pinea, the stone pine does similar things and it could be seedlings of that instead.