Winter hardy, half hardy and not hardy - what is what?
If you enjoy sowing, you have probably come across the terms hardy, half-hardy, or tender. But what do they actually mean, and more importantly: what should you do with them if you want to start sowing yourself? I’m happy to explain.
Hardy annual
Hardy annuals are actually the strongest of the group. They can easily withstand our winters, and many of them happily self-seed. Sometimes they even germinate in the autumn, and if the winter is not too wet or harsh, those little plants can survive the whole winter. However, most species wait until the spring sun warms the soil again, and then they often all emerge at once — a delight to see.
You can sow these types both in autumn and very early in spring. In autumn, they often germinate in the warm soil and go into winter as small plants. The only thing to watch out for is that they do not grow too large before the frost period, as this makes them more vulnerable. If you prefer to sow in spring, that’s fine too: many hardy varieties benefit from a cold period, as this helps them come out of their dormant state. Once the weather gets warmer, they shoot up quickly, and you are assured of flowers extra early.
But — and this is something many people encounter — not all hardy annuals perform equally well in open ground. If the spring is very wet, there is a chance that only the strongest will survive. Therefore, for species that do not like wet feet, it is often better to sow them in cardboard pots, egg boxes, or preferably in seed trays.
This gives you a lot of control: the soil is not compacted, birds and cats stay away, and slugs find them less quickly. Covering with a thin layer of coarse sand or gravel also helps to prevent moss growth. All these pots and trays can simply be left outside, even in frost or snow. If necessary, you can protect them during prolonged rain by placing a glass sheet over them, or by putting them on a light, sheltered terrace. This way, they grow calmly and naturally, without the risk of cold, muddy garden soil. By the time you can transplant them, they will have a significant head start on weeds and will be strong enough to withstand a bit more competition.
some hardy species
- slaapmutje (Eschscholzia californica)
- mejuffertje in 't groen(Nigella damascena)
- korenbloem (Centaurea cyanus)
- ridderspoor/ Delphinium consolida
- klaproos (Papaver rhoeas)
- gipskruid(Gypsophila elegans)
Not hardy annuals
Non-hardy species are the truly frost-sensitive plants. They sometimes self-seed, but as soon as the frost arrives, the young plants die or the seeds often remain inactive until after the last night frosts. If the seeds have not been frozen, they dare to germinate, as is the case with Indian cress, for example.
In principle, you can sow these types outside as soon as it no longer freezes, but that doesn't always work well. Some types take so long to go from seed to flower that you would only have flowers in August or September — and that is often not the intention. Other types, especially tropical or subtropical, require a lot of warmth to even germinate.
Therefore, non-hardy annuals are almost always sown indoors or under glass. This can start as early as the end of January or the beginning of February for the slowest growers, but it is better to wait for a period of clear, sunny days. Light is at least as important as warmth. A greenhouse or conservatory is ideal, but a bright windowsill can also work — as long as the seedlings receive enough light and you can later move them to a cooler spot. As soon as the first shoots appear, the glass cover must be removed immediately so that the seedlings get fresh air and grow sturdy. You want them to have "warm feet and a cool head": roots in slightly warmer soil, but the foliage in cooler air. This prevents them from developing long, weak stems.
If you don't have a greenhouse, you can let them germinate indoors and put them outside during the day as soon as the weather permits, possibly under a glass cover. As temperatures rise, they can occasionally stay outside at night as well. This way, they become much stronger than if you try to grow them indoors with central heating. By mid-May, after the Ice Saints, they can be permanently moved outside.
some non-hardy species
Tithonia rotundifolia
Afrikaantjes (Tagetes)
spaanse vlag (Quamoclit lobata)
Statice (Limonium sinuatum)
Dahlia’s
Halfhardy annuals
And then there is this intermediate category: the half-hardy annual plants. Many of these plants come from warmer regions and are actually perennials there, but here they do not survive the winter. They can tolerate a bit of cold, but absolutely do not like extreme weather conditions, especially not a waterlogged soil in spring. Moreover, they often require a longer growing season to come into full bloom. If you sow them in the open ground after the frost, their flowering usually starts too late.
VIn the past, glass cloches were used to sow these plants on-site while still protecting them from wind and rain.That still works, but is not always practical. A cold frame or a sheltered overwintering space is much easier and gives you a lot of control. These types do not require soil warmth, but do need a lot light. Sow them in containers, pots, or trays, depending on how well they tolerate transplanting. Allow them to grow cool but in light, and later transplant them into slightly larger pots until they are strong enough.Here too, the rule applies: harden them off well before they go outside. From mid-May, they can be planted in their final spot with their root ball, giving them a significant head start against weeds and slugs.
some half hardy species
Leeuwenbekjes (Antirrhinum majus)
Afrikaantjes (Tagetes)
Statice (Limonium)
Verbena
Finally
Sowing is primarily a matter of getting to know your plants. Each category has its own preferences, and if you take that into account, the whole process becomes much easier. And believe me: once you get the hang of it, it's truly delightful to see how differently those seedlings respond — the tough hardy ones, the warmth-loving tender ones, and the sensitive half-hardy ones in between.
Two-year-olds naturally belong to the category of plants that can be sown outside without any problems and that will subsequently self-seed at the most suitable moment for them.
Biennials are best sown on site in June and July, to form strong rosettes before winter, from which flowering will emerge the following year, starting in May. Alternatively, one can also sow in pots, trays, etc., and later transplant the plants on site in September/October, with minimal disturbance to the roots. In the following years, biennials will usually self-seed, more easily on lighter soils than on heavy, clayey soils.