DARD Management Notes: Horticulture

Before planting a cut flower crop in a greenhouse take a soil sample
Before planting a cut flower crop in a greenhouse take a soil sample

Protecting horticultural crops against cold weather: Don’t let the unseasonably mild weather experienced during December make you complacent about protecting your horticultural crops this winter.

Remember the winter of 2010 when Northern Ireland experienced an overall average temperature of -0.6 degrees centigrade which was the coldest December on record. These extreme sub-zero temperatures caused wide spread damage to commercial horticultural crops and nursery irrigation facilities.

Plants in containers are more likely to suffer winter damage than field grown plants. The root system of container grown plants is above the ground and exposed to much lower temperatures than in the soil. Freezing damage occurs when ice crystals form within plant tissue and rupture the cells. Research has shown that roots can be killed on subjects such as Ilex (Holly) at -4.0 degrees centigrade. A lot of plants initially appear to have survived the sub-zero temperatures but collapse when growth starts the following spring. Desiccation or drying occurs because water uptake by the roots is exceeded by loss from the leaves and stems. This happens when the root ball is frozen and the air temperature begins to rise. The effect is made worse by cold winds. As we did not experience cold winds in 2010 at the time of the low temperatures, frozen root damage did not show on many plants until growth restarted in spring.

The extent of winter damage can vary according to location of an individual nursery or farm and between regions. Minimise the risk of damage to your horticultural crops and equipment by:

* Using appropriate insulation and frost protection measures

* Bringing plants under protected structures and using wind breaks

* Covering plants with protective fleece materials

* Careful watering of plants, to avoid overwatering

* Draining out irrigation systems and equipment before cold weather sets in

* Setting thermostats to give a regime -1.0 degree to +5.0 degrees centigrade

* Soil sampling of glasshouse soils

* For protected crop growers the New Year is an appropriate time to take a glasshouse soil sample before new season crops are planted. Annual soil sampling is important for soil grown glasshouse crops to check pH, nutrient and Electrical Conductivity (EC) levels. Sample outdoor areas every four years but always sample new ground.

When taking a soil sample use a soil auger to take cores from the root zone, normally 150 mm deep. Take cores from all areas of the glasshouse, normally in a ‘W’ pattern. Sample separately any uncharacteristic areas, for example workstations or entrances. Remove any crop debris or plant material in the sample. Place the soil cores into a clear plastic bag with name, date, name of crop and greenhouse number. Samples should be 500 g in weight otherwise it may be rejected. Store at a cool temperature before posting to laboratory for analysis.

The glasshouse soil analysis will state nutrient levels of nitrogen (ammonium and nitrate), phosphorus, potassium and magnesium as an index. Optimum nutrient levels will vary according to the crop. For cut flower crops a target pH of 6.5 will ensure the maximum availability of nutrients for plant uptake. If the analysis states a high EC level (‘salts’), for example above Index 3 for cut flower crops, remediation will be required before planting to prevent crop losses.

Before selecting a laboratory to carry out the analysis, check if the cost includes interpretation and length of time to receive results. Contact CAFRE’s Cut Flower Adviser for guidance on taking soil samples or interpretation for cut flower crops.