Notes on selected genera and other groups
The raising of most seeds is easy if the correct methods are followed. A few, however, can sometimes be more of a challenge and additional advice on some of these, together with notes on the main seed groups, are given below.
Carrots and Parsnips
Don’t be in too much of a hurry to sow these if the soil is cold and wet and never use old parsnip seed from a previous year. Germination tends to be slow and a few fast-germinating radish seeds may be added to mark the rows. Avoid sowing on ground that has been recently manured and on heavy, shallow or stony soils grow one of the short-rooted varieties.
Cineraria (indoor varieties)
These are sown in spring or summer for use as winter-flowering pot plants. For flowering around Christmas they need to be sown in April or May. Good germination is usually easily achieved by following the same methods as for half-hardy annuals but successfully growing the plants on through the hot summer months requires a little more care. The trick is to keep them as cool as possible and, in warm weather, a shaded and well-ventilated cold frame is usually the best place for them. Aim to keep them moist at all times but never waterlogged. Pot them on as they grow but don’t rush this as over potting can result in plants that are excessively leafy.
Seeds should be sown on edge and require a temperature of 21-27ºC (70-80ºF) to germinate well. Expensive seeds may be pre-germinated or ‘chitted’ before sowing. This usually involves placing them on moist blotting paper in a plastic sandwich box and then planting them individually in small pots as soon as the first signs of germination are seen.
Greenhouse varieties, particularly the all-female types, demand warm conditions throughout their lives. For an unheated house, ordinary varieties such as Telegraph are usually the best and even they should not be planted out before late May in most areas. As the seedlings grow rapidly, sowing in the second half of April is early enough for a cold house.
Half-hardy annuals (flowers)
This is the largest group of flowers we grow and includes most of those we refer to as ‘bedding plants’. All have been developed from plants which grow in warmer climates than ours and although, given reasonable weather, they are perfectly happy in our gardens during the summer, they are unable to survive frost. Some can be direct sown in the garden if the sowing is delayed until the soil is warm in late April or May but, for best results, all should be sown rather earlier than this under glass. Some half-hardy perennials, such as begonias and geraniums (pelargoniums), are commonly also treated as half-hardy annuals and grown in the same way.
These differ from half-hardy annuals in that they are able to survive moderate frosts and can all be sown direct in the garden, either in spring or, in some cases, in the autumn. Most dislike being transplanted and are sown where they are to flower. A few may also be treated as half-hardy annuals and sown under glass to encourage earlier flowering.
Hardy biennials and perennials (flowers)
Whether they live for only two years (biennials) or many (most perennials), these are commonly direct sown in a seedbed in late spring/early summer (about June) and transplanted to their permanent positions in autumn to flower the following year. Varieties with very small or expensive seeds may instead be germinated in seed trays located in a sheltered position, such as an open cold frame, and the resulting seedlings later transferred to a nursery bed to grow on.
A number of hardy perennials will flower in the first year, if sown early enough, and these are often raised under glass in the same way as half-hardy annuals. They are often referred to as “first year flowering perennials”. Conversely, some perennials, particularly most trees, shrubs and bulbs, but also some herbaceous types, are slow to develop and may require two or more years growth before flowering. A few hardy perennials, such as pansies and some dianthus, are commonly treated entirely as half-hardy annuals and only retained for a single season.
These seeds require a period of cold to germinate successfully and should usually be sown in the autumn following the method described under ‘seeds which require cold’. Rather than placing the pots or trays immediately outside, however, we have found that best results are often obtained if they are first kept indoors for about 4 weeks at a temperature of around 21ºC (70ºF).
Whether stood outside directly or given an initial warm treatment of this kind, germination should start in late winter. As soon as the first signs of this are observed, bring them inside to a position where a temperature of about 10ºC (50ºF) can be maintained and prick out the seedlings into individual small pots or cells as they become large enough.
A number of more or less tender annual and perennial plants, particularly some flowers which are commonly grown as house or greenhouse ornamentals, are also raised from seed. Some of these require a little extra care with potting on and watering if fully satisfactory results are to be achieved but the technique of raising seedlings is essentially the same as for half-hardy annuals. In the frost-free indoor environment, the time of sowing of some types can be varied according to when plants are required to flower. A few can be put in at more or less anytime but, as a general rule, most are best sown sometime in the spring so that the developing plants have the benefit of the higher light levels at that time and through the summer.
All the leafy brassicas (Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Kale) are normally sown in a seedbed and the resulting seedlings later transplanted to their final positions. Both the seedbed and the planting area should be limed if the soil is acid, as brassicas do not grow well under acid conditions. They also need firm ground so only very light surface cultivation should be carried out before sowing or transplanting. For an early start, seedlings of some varieties may also be raised under glass where the germination temperature should be about 15°C (60°F).
Among the more tricky of plants to grow from seed and not recommended for the novice, Himalayan poppies are such exceptionally beautiful plants as to be more than worthy of the extra effort needed for success.
If you have purchased seed well in advance, keep it in an airtight container in the bottom of your refrigerator to avoid loss of viability. For sowing use an acidic, lime-free or ericaceous, soilless compost; ordinary general purpose and coir composts are not suitable as they are usually too alkaline. Sow in late winter to spring, preferably in February, as this allows the young plants to germinate and become established before the sun becomes too intense. After sowing on the surface of the compost cover the seed with a thin layer of seed-sowing grade vermiculite and then cover the pot or tray with a sheet of glass. Do not add any other cover as light is required for germination. Make certain that the compost remains moist at all times and turn the glass occasionally to prevent a build-up of condensation. Germination can be very erratic but under these conditions should start within 3 or 4 weeks.
After germination prick out the seedlings into cells, again using an ericaceous compost. As Meconopsis seedlings are very vulnerable to drying out, it is important to maintain humidity until they have re-established, such as by covering them with a propagator lid. As they are also very susceptible to scorch they must be protected form direct sun and excessive temperatures while under glass. As soon as the are large enough they should be hardened off and transferred to a shady cold frame. Avoid watering with tap water if you live in a hard water area. Rainwater can be used as soon as plants are well established but not for seed trays or young seedlings as damping off may then become a serious problem.
Primula (including primroses and polyanthus)
The main problems with germinating primula seeds are that they tend to be slow to start and also require both cool conditions and light. They are subject to a condition known as heat-induced dormancy and may not germinate at all if temperatures rise above 20ºC (68ºF); about 15ºC (60ºF) being about right. For this reason, avoid using a heated propagator. Sow all varieties on the surface of soilless compost and then cover very lightly with seed-sowing grade vermiculite. Cover each tray or pot with a sheet of glass and do not allow to dry out at any time. When the seedlings start to emerge remove the glass.
Our native species germinate best if the seeds are subject to a period of cold. They are, thus, best sown outside in the autumn but still using a soilless compost, as above. Other species and varieties can generally be sown under glass from January to March, as long as sufficient ventilation can be given to keep temperatures within the required range on sunny days. However, the safest method, in most cases, is to delay sowing until between March and June and then to do so in a shady cold frame. Species such as P. beesiana and P. cockburniana may give better germination if refrigerated for about 3 weeks before sowing, using the method described below.
Seeds which require cold
Most seeds ripen in late summer or autumn and, to ensure they don’t germinate until more favourable conditions return in the spring, those of many trees, shrubs and alpines, a few other garden plants and some wildflowers such as the primrose and cowslip, will remain dormant until they have experienced a period of cold. Such seeds are normally sown outdoors in autumn to germinate in spring and, in this instance, sowing is best done in a seed tray or small pots, in most cases using the soil-based John Innes seed compost. To prevent growth of mosses and liverworts it is a good idea to cover the surface of the compost with a fine layer or horticultural grit and, when sowing is complete, the trays or pots should be placed in a sheltered and shady place such as a cold frame or against a north wall. In spring the can be brought into a greenhouse or conservatory where there is some gentle warmth in order to hasten germination.
An alternative to the above is to mix the seeds with a little moist sand or vermiculite and to seal them in a plastic bag. This is then placed in the bottom of a normal domestic refrigerator, usually for about 6-8 weeks, after which it is removed and the mixture sown using normal ‘under glass’ methods. The storage of seeds under cold conditions to overcome dormancy is termed ‘stratification’.
Good soil preparation and plenty of water in dry spells are essential for this crop. It will not succeed and the plants will often flower prematurely (‘bolt’) if the soil is poor or they become dry at the root. A good place to sow is in the dappled shade between rows of taller growing vegetables.
There are three sowing periods for sweet peas, autumn (September to October), late winter (January to February) and spring (March to April). Autumn and winter sowings give the best results and are made under glass, although autumn sowings can also be made in a cold frame. Spring sowings are made direct where the plants are to flower.
Under glass use a soilless compost and 12cm (5in) pots, sowing 5-7 seeds in each pot. Peat pots and rootrainers are also suitable. Sweet peas are not difficult to germinate and most will grow perfectly well if just sown in the normal way. Varieties with hard, black or mottled seeds may, however, sometimes do better if chipped or rubbed with sandpaper. Brown or wrinkled-seeded varieties should not be chipped.
Sweet pea seedlings need cool conditions at all times, if they are to make sturdy plants, and autumn sown plants in particular need to be grown hard so they do not develop too quickly. Seeds will germinate happily at 12-15ºC (55-60ºF) and, once the seedlings are through, they should be kept in a cold greenhouse or transferred to a cold frame. During periods of hard frost some extra protection should be provided, such as covering with fleece, but no heat is required and maximum ventilation should be given whenever weather conditions allow. Frames should only be closed when there is frost or heavy rain.
Our native wildflowers may be hardy annuals, biennials or perennials and their basic requirements are much the same as for other plants in these groups (but see under Primula for Primrose and Cowslip). Some, such as Foxgloves and Primroses, make excellent border plants and most can be readily raised in seed trays or beds using the methods described for garden flowers, although germination is rarely as rapid or as uniform as with garden varieties. Mixtures of annual and perennial types are also often direct sown with grasses to create wildflower meadows.