Growing your own giant sequoia
The answer is: yes you can, provided you're living in a temperate climate zone.
More about the world regions where giant sequoias have been planted successfully, can be found here.
But you have to keep in mind that giant sequoia (Sequoiadendron giganteum) are not fit for small city gardens. This might not come as a surprise but I have seen this tree planted very often in locations that were not well suited, as this species is a vigorous grower and can reach a height of 10 m (30 ft) after 10 years, and reaches a height of 30 m (100 ft) to 40 m (130 ft) after 50 years. A giant sequoia is not fit to plant as a small ornamental tree, but reaches its full potential as a landmark tree that can grown without restraints.
Before you enthusiastically imagining gigantic scenes: please keep in mind that, although the giant redwood is a vigorous grower according to plant terms, this species is living at another pace as you are. When you'll be, hopefully, an elderly person, your giant sequoia will only have grown through his first toddler years. In one human life the giant sequoia is able to reach a girth of 4 to 5 m (13 to 16 ft). Do you want to plant one anyway? Please go ahead and think about the next generations, that will hopefully be able to see this tree as a true giant.
Where to get your seeds?
To grow giant sequoias from seed, first thing you need to do of course is to get some seeds. There are two ways to do that. First you can buy them from nurseries/seed companies. For the Europeans amongst you, there are a number of good British or German nurseries that offer Sequoiadendron seeds (the tree is sometimes called Wellingtonia in British English and Mammutbaum in German).
More fun to do as a home gardener, is to collect the seeds yourself.
- Bought seeds should originate from mature trees in the natural range of the trees, the Sierra Nevada range in California, and are the most likely ones to germinate.
- Trees planted in Europe, Eastern US, Asia, and South America are not yet as old as the old growth Californian redwoods. The oldest ones are only now starting to make the transition from toddlers into teenagers and are only now starting to produce viable seeds.
So when you collect them yourself, try to collect cones from trees that are as old as possible.
The image above shows a cone of a Belgian redwood, the giant sequoia of Esneux, which is largely bigger than the cones of trees with a girth below 6 to 7 m (about 20 feet).
The giant sequoia is monoecious, which means that the male and female parts are located on the same tree. So for pollinated, viable seeds you only need one tree, it's not necessary that there are other giant redwoods in the immediate neighborhood.
It can take only a couple of years to a number of decades before a giant redwood starts producing cones. I have seen sequoias only 1.5 m (5 feet) tall with an abundance of cones and trees up to 20 m (about 60 feet) tall, with no cones at all. Probably they are forced to produce cones after a stress period, such as a long period of drought, as survival mechanism.
Fallen green cones are also ideal to collect: when they open after a couple of weeks in a dry place, they are loaded with small, winged seeds.
The biggest cones also produce the biggest seeds. On the left you can see some seeds from the Belgian giant sequoias of Oostmalle, on the right those of the one of Esneux, mentioned above. There's a distinguishable difference in size (scale in cm).
Before sowing, it might be useful to place the seeds a couple of days to a couple of weeks in the fridge. When you sow them at last and place them in a warm place (like on the radiator of your heating system), the seeds "think" winter's over and the time to germinate has come.
I have tried the seeds at different depths and had the most success when I did not put them in the (ordinary) compost, but on the soil, not to only very slighty covered. Probably they also need light to germinate. But be aware! The seeds need to be in (intense) contact with the moist soil, so you need to press them softly into the compost. The seeds are quite susceptible to drought. You can put a glass plate or some plastic foil over the pot, but you have to be careful not to kill them by making things too wet. Small germinated sequoias die rather easy because of overwatering. I can tell you: it's a very sad thing to see baby sequoias wither away!
To keep the ground moist but not wet, it's ideal to use a garden sprayer instead of a watering can.
You can sow them all year round, but because the plants in temperate regions like Europe will grow best in summer, it might be best to do the sowing in (early) spring. Seeds that germinated here (Belgium) around Christmas, have not grown noticeably until spring.
How long does it take the seeds to germinate?
Well, first of all it should be said that giant sequoia seeds have a very low chance of germinating. If a few from some twenty to fifty seeds germinate, you're already successful... With bought seeds, the germination rate is higher but it's highly probable that more than half of the seeds will do nothing. Readers of the website who already tried this, have had germination rates of 15 to 25%.
The seeds germinate at the earliest a couple of days after sowing, but still can after months. Don't think things will not work out: patience is a virtue!
When a seed germinates, the first thing you see is a tiny, rhubarb colored stem, loop shaped. After a couple of days the plant erects itself and the dried seed skin falls of (if it does not look like it's going to do that by itself, you could help a little bit). You see four seed leaves, although there can be three or five (like on the image below).
On this image, the baby giant sequoia is a week and a half old. The height is about 2 cm (an inch). The first, careful steps in the life of a beautiful and fascinating tree, that is likely to become a giant that will outlive me multiple times.
Growing giant redwoods (Sequoiadendron giganteum) from cuttings is also possible, but I do not have much experience with that. Once I managed to get a few through the winter. When they started growing new green tips in spring, I planted them in the hope that they would root, but sadly that didn't work. The use of a rooting hormone like IBA, adjusted compost, and some more trial and error experience would probably dramatically increase the chance of success. When you're striking cuttings, please do so late in season when the growing of the new offspring is still strong, but the wood has already hardened somewhat.
You should also strike your cuttings from trees as young as possible. For the Europeans/Aussies among us, the older trees could be used too, since all planted trees are not "old" yet.
The different horticultural varieties or cultivars are all propagated by grafting.
Also the variety 'Pendulum' is not very hard to find. Specialized tree nurseries have larger trees available, but those can be very expensive.
The two other redwood species
The two other sequoia species, the coast redwood (Sequoia sempervirens) and the Dawn Redwood (Metasequoia glyptostroboides), can be grown from seed more easily (more than half of the seeds germinate). Nevertheless these species can be propagated best (and definately the dawn redwood) by making cuttings. In contrast to the giant sequoia, these cuttings root easily to very easily.