This is the month of many tasks which will brook no delay, for growth now becomes so active that very soon it will be too late to transplant or to prune many kinds of plants without risk of injury. Seed sowing, too, must be carried on in greater measure than during preceding months. It will be well to look carefully through the earlier sowings, and should any batch of seedlings show poor promise of success it will probably be wiser to sow again without delay than to wait on the offchance of improvement in checked and stunted seedlings.

Although it is time now to finish moving most deciduous trees, shrubs and climbers, and a good many of the more forward herbaceous plants, April is a good month for planting conifers, hollies, laurels and other evergreen shrubs and hedging plants. Gladioli, also, will be better under the soil than out of it, although, where a long session of bloom is desired, a proportion of the corms may be held over for planting next month.

More often than otherwise we have found April pruning of roses productive of better results than those obtained by cutting back in March. That topmost buds have broken into leaf is of small account; the important point is not to encourage the best of dormant eyes to burst into soft growth too soon. Remember we sometimes have fairly sharp frosts in May, followed by bright sunny mornings, causing rapid thawing, and it is that which bursts soft sappy cells and irreparably injures fine young growths.

This is a great month for real spring flowers.Frost may stamp its foot with utter finality on the beginning and end of some of our gardening activities, but absence of frost is not the only requirement for good growth. Plants need energy to grow, and they receive some of this in the form of warmth which is why we use our green houses to grow our bedding plants and to give some vegetables an early start. In the greenhouse we can of course provide artificial heat but the plants in the garden must rely on warmth from the air. Some indication of warmth is given to us in a simple form by the air temperature. Temperature and warmth are not the same thing, but the do walk hand in hand together and rarely stray far apart. We cannot measure temperature adjacent to all our plants so an indication of a reasonable temperature indicated by a thermometer on a sheltered wall is not a measure of the warmth being enjoyed by a plant in a draughty place in the garden.

The mean air temperature 42 or 43 degrees F (5.5 or 6 deg C) has often been stipulated as a threshold temperature, below which growth of plants does not take place. At best this is likely to be an approximation which will not be true for all plants or in all types of temperature variation. On the other hand, it does not seem to be far from the truth for the growth of a plant such as grass, and maps showing the mean date that the average temperature rises above 42 deg F (5.5 deg C) do seem to indicate something like the ‘beginning of the growing season’. Such maps, to be of any use, must show actual temperatures, and not temperatures which are reduced to sea level. Meteorologists often reduce physical quantities to an arbitrary uniform level, because the effect of height would swamp all other variations. A farmer or a gardener, on the other hand, is concerned with the true temperatures at the actual height as no garden can be reduced to sea level even on a map.

Taking this temperature as a base, and adding together the successive products of the time in days and the excess temperature above the base, we obtain something called ‘accumulated temperatures’. For example, one day with a mean temperature of 52 deg F would count as 10 day-degrees (52 – 42 = 10). Many attempts have been made to correlate such sums with plant growth or with time of blossoming, and in some cases an air of verisimilitude has been suggested by re-naming them ‘accumulated heat units’ which is just what they are not. Temperature is not heat and having said this it is not unreasonable to ask what measurements a meteorologist does in fact make of the all important supply of heat energy. This energy arrives by process of radiation from the sun and he attempts to measure it by using an instrument which tells him the duration of bright sunshine. The sunshine recorder he uses has severe limitations, but is simple and robust and is far more helpful than some would have us believe.

We must remember that the sun’s rays are stronger in the south than in the north, although the longer summer days in northern latitudes almost compensates for this effect as far as maximum total heat per day is considered. In winter the north has both shorter days and weaker sunshine so the latitudinal effect is twofold. For simplicity we will consider only ‘sunshine hours’ – the longer and stronger the sunshine, the greater is the supply of energy to a plant.

Returning to ‘accumulated temperatures’ those above or below any suitable threshold are very useful for problems such as the heating of greenhouses or the storage of food, but they are of limited validity in questions of growth so we need to look elsewhere for some indication of relative warmth. A factor which suggests itself is the day maximum temperature (which is readily obtainable by all of us via a maximum and minimum thermometer) and the average date when the day maxima reach over 50 deg F (10 deg C) might be an acceptable indicator of the beginning of ‘spring’. It is difficult to find this date directly, but we can easily find the date when the average day maxima exceed 50 deg F, which is almost the same thing. For example, if the average day maxima at any one place for the month of March are 50 deg F, then the date we are seeking is the middle day of March i.e. 15th to 16th. Before that date there is less than an even chance of getting a day maximum above 50 deg F; after that date the reverse is true, and there are more warmer days above 50 deg F than cooler days below 50 deg F. On the mean date March 15th there is an even chance of the ‘spring’ day.

In an ‘average’ year we might expect the earliest date of ‘spring’ to be March 1st in southern Cornwall. During the next eleven days or so ‘spring’ has crept rapidly inland over about half of southern England, omitting Salisbury Plain, north Hampshire, Surrey, Sussex and Kent. A similar state of affairs exists in the Channel Isles and in southern Ireland, where most inland districts of Tipperary, Limerick, Kerry, and Cork have crossed the ‘borderline’. The southern Irish coasts are still kept cool during the daytime by the deep Atlantic, which may help to ward off the winter frosts and yet does not encourage high day maxima.

Ten days later ‘spring’ has, on average, spread to all England south of a line between the Wash and the Mersey except the coastal strip of East Anglia and south east England where the North Sea is still cold enough to keep the temperatures low. In Wales the coastal areas of both north and south are included, but the higher ground inland and much of the Welsh coast are still cool by day. In Ireland, only Ulster and Donegal are outside the March 21st line. By the end of March such ‘spring’ days are normally reached over all England and Wales except the Pennines, the Lake District hills and Northumberland. In Scotland, however, only the west coast south of Skye is included and it is not until May 1st that we reach the average date for the Caithness area.

The colder than average weather we have experienced this winter means that we cannot expect ‘Spring’ to arrive by mid March. There is every indication that things in general are some 4 to 6 weeks late – the Christmas Cactus in my porch is normally in full bud when I install the Christmas Tree but at the time of writing in early March it is not yet in full flower. We normally see crocus in flower in mid January but they are only now coming into full bloom. On this basis we should not rush into early planting as the ground is still very cold and the recent very heavy rain has not been conducive to the completion (or even the starting) of ‘winter digging’ in some areas.