Gadenin日本語Get the appCold is a requirement, not a hardship
Stock is a cool-season plant that will not initiate flower buds until it has experienced a spell of low temperature while it is at the right stage of growth. In practice it needs roughly two to three weeks with temperatures largely between 40 and 55F (4 and 13C), starting once the plant has made about four to six true leaves. Below that size the plant is too young to respond; well past it, the response weakens.
This is why the same seed produces spectacular plants for one gardener and nothing but foliage for another. A sowing that goes out early and sits through a cool spring gets its cold spell without anyone arranging it. A sowing that goes out late into settled warm weather never gets it, grows fast, looks healthy, and refuses to bud.
Cold is also why an autumn sowing in a mild-winter garden gives the best plants of all. Those plants get a whole winter of cool temperatures at exactly the right size, and they respond with the longest, heaviest, most strongly scented spikes the plant is capable of. The trade is that the same conditions will kill them where winter goes below about 20F (minus 7C) without protection.
| What the plant experienced | What you see | What to do differently |
|---|---|---|
| Two to three weeks at 40 to 55F (4 to 13C) at four to six leaves | Full spike, heavy scent | Nothing, repeat the timing |
| Warm throughout from sowing | Tall leafy plant, no bud at all | Sow four to six weeks earlier next time |
| Cold arrived before four true leaves | Weak or partial spike | Sow slightly later so size and cold coincide |
| Cold spell, then early heat | Short spike that browns quickly | Site in afternoon shade, sow earlier |
| Overwintered small plants, mild winter | Longest spikes, earliest flowering | Repeat, and protect from hard frost |
Heat ends it, and ends it quickly
Once night temperatures settle above about 65F (18C), stock stops. Buds that have already formed will open, but the florets are smaller, they brown at the edges within a day or two, the scent thins, and no new buds are set. A plant that was magnificent one week can look finished ten days later, and there is nothing wrong with it.
You can buy a little time. Afternoon shade during the hottest part of the day slows the decline noticeably, and so does keeping the roots evenly moist, because a plant that dries at the root in heat gives up immediately. Containers can simply be moved into shade for the afternoon. But these measures extend the season by two or three weeks; they do not change the fact that this is a spring and early summer plant in most climates, and an autumn and winter plant in hot ones.
In a mild-winter, hot-summer climate the sensible pattern is to treat stock as a cool-season plant grown right through the winter, sown in late summer, flowering from late winter to mid spring, and pulled before summer heat arrives.
Doubles, singles and what opens on the spike
Even in a garden where the cold spell has landed perfectly, about half the plants of a doubles strain open as single flowers with four flat petals, and about half open as doubles with a full rosette of petals. This is genetic and nothing you do to the plant will change it. The doubles hold their flowers far longer, are more strongly scented, and are what most people are actually growing stock for.
The singles are not useless. They are the only ones that set seed, since fully double flowers have no functional reproductive parts, and they still carry scent, just less of it. If you have no interest in saving seed, pull the singles as soon as they show and give the doubles the room.
Since the two kinds can be told apart as seedlings by their paler, larger leaves, the practical answer is to sort at the tray stage rather than being disappointed at flowering. A bed of stock in which every plant is double looks and smells like a different plant from a bed that is half and half.
A single-flowered spike opening among plants you sorted as doubles is normal. No sorting method is perfect, and the trait is not fully separable in every strain.
The evening is when it performs
Scent output rises through the afternoon and peaks in the first hours after sunset, when air movement drops and the flowers release most heavily. On a still, mild evening a group of a dozen plants will scent an entire small courtyard. On a windy afternoon the same plants can seem almost unscented, which leads people to conclude that their strain is a poor one when it is simply the wrong time of day.
Judge your plants at dusk, standing still, out of the wind. If they are strongly scented then and disappointing at noon, they are performing normally. If they are weak even at dusk on a still evening, look first at how many of them are single-flowered, and second at whether they have been fed heavily with nitrogen, which produces lush plants and thin scent.
After the spike is over
Branching kinds respond to having each spent spike cut back to a leaf joint by producing side shoots, and with cool weather and steady moisture they will give a second and sometimes a third flush. Single-spike kinds have given everything they have; cut them for the house and then pull them, and use the space for something that likes the heat that is coming.
If you want seed, leave two or three single-flowered plants standing until the long, slim pods dry to pale brown on the stem. Seed saved from your own plants will still throw both doubles and singles, in a similar ratio, so the sorting exercise starts again the following spring.
Whatever you do, note the date the first spike opened and the date the last one browned. Two seasons of those dates will tell you more about the right sowing time in your own garden than any general recommendation, because it turns the abstract question of when the cold spell falls into a number you can count backwards from.