Potatoes

Overview

Potatoes – Solanum tuberosum

Potatoes are a popular crop because they can be grown on a wide range of soils. They prefer a sunny open site that has a deep, well drained, fertile loam with a p.H. of 5.o-6.o to do best. It doesn’t matter if the soil p.H. reading is slightly higher but any lime in the soil encourages potato scab which isn’t a serious problem.

Potatoes are a labour intensive crop because they require earthing up several times during the growing season and digging them up can take up several hours sometimes spread over days. They are a heavy and bulky crop to store. All of these things have to be taken into consideration before committing too much of your time and allotment space to potato production.

Potatoes are grown from seed potatoes. The ideal seed potato should be about the size of a hen’s egg and sit in the palm of your hand nicely. Most of the seed potatoes offered for sale are grown in Scotland where the risk of the plants contracting any of the various potatoes diseases is minimal. They are sold as clean Certified Seed Potato Stock. It is a very risky business saving your own seed potatoes or to accept gifts from fellow gardeners. Never be tempted to use the cooking potatoes offered for sale as seed because they could be carrying diseases as well.

Varieties to choose (all sold by Kings Seeds)

 

How to grow

Chitting          

When you buy seed potatoes they will be offered for sale either loose from a hessian sack or in pre-weighed plastic net sacks. Remove the seed potatoes from any packaging as soon as possible, inspect them and remove any that are damaged or diseased. If you look closely at the potato tubers you will see a cluster of “eyes” at one end; this is referred to as the rose end and at the opposite end is a single scar where the tuber was joined to last year’s seed potato. In order to improve the weight of the crop the seed potatoes have to be kick started into to growth using a process known as chitting. This involves standing the seed potatoes vertically in a seed tray with the scar end at the bottom and the rose end at the top in. Place the tray somewhere frost free and in good light to encourage  the eyes to sprout and develop into short green shoots that will produce the potato crop during the summer. Don’t confuse chitting with forcing; all that the seed potatoes require at this stage is good light and a temperature around 60F/ 16C. The chitting process fits conveniently into the period leading up to planting out time from March until the end April.

Seed potatoes are usually available from late January onwards it is important to buy your stock as soon as possible because the longer that they remain in sacks they will deteriorate; either rotting or producing long white shoots that are useless.

 

 

General

Although potatoes aren’t frost hardy they can to be safely planted out from early March onwards. They will be protected from any frost damage under the soil and the top growth (haulm) usually emerges above the soil as the threat of frost decreases. Sometimes a frost will damage the haulms but if you can sprinkle water over the frosted foliage before it thaws it will help the plants to recover.

Planting

Potatoes are organised into three groups; early, second early and maincrop or lates these terms refer to the lifting time not the planting out time. They reflect how long it takes the potato crop to mature and become ready for lifting;

Depending upon your local climate try and plant out all of the seed potatoes by the end of April to give them the longest possible growing season, sometimes this isn’t possible and it will be reflected in the weight of the crop.

Before the seed potatoes are planted all but three of the chitted shoots must be rubbed off. This is to allow the plant to produce a reasonable crop of usable potatoes. If all of the shoots are left on you will get lots of small potatoes. If you want chipping or baking potatoes leave two eyes.

Earlies are planted 6ins/15cms deep in rows 12ins/30cms apart with 24ins/60cms between the rows. The most common method is the take out as trench that has one vertical side to support the seed potato before filling in the trench with the loose soil taking care not to disturb the seed potato. If the soil is in good condition the seed potatoes can be planted using a trowel. They will be ready for lifting from mid-June.

Early potatoes will not store well and must be used as soon as possible. They are best lifted as required for the table.

Second earlies are planted 6ins/15cms deep in rows 18ins/45cms apart with 30ins/ 75cms between the rows. Second earlies are ready for lifting from August they will be ready to use until the end of October.

Lates or Maincrops are planted out 6ins/15cms deep in rows 24ins/60cms apart with 30ins/45cms between rows. Late/maincrop potatoes are ready for lifting from October onwards they will keep in store until Spring of the following year. Practically leaving the potato crop in the soil until this late in the season is risky it can cause all sorts of problems. The most dangerous is potato blight which is always around at this time of the year. It will destroy the crop and all of your hard work would have been for nothing. Secondly the weather can be very wet around this time of the year making the soil too sticky to walk on this usually ends up with the crop rotting in the ground.

The best to plan is to treat the entire crop as second earlies and aim to have lifted all of the potatoes by the end of September.

Growing on

Potatoes grow along the stem of the haulm as it extends. If the developing potatoes are exposed to daylight they will turn green and become unusable it is for this reason that keeping in the dark by covering them with soil is such an important job. The earthing or the rows up helps to increase the potential weight of the crop by increasing the cropping area.

Green potatoes are poisonous they must be discarded.

Earthing up

Once the haulm is about 9ins/20cms tall it is ready for the first earthing up. Draw the soil up along the row using a draw hoe or spade leaving just a little of the top leaves visible out of the soil. Wait until the growth has developed by another 9ins/20cms and repeat the process if it is possible carry out this operation once more. The final ridge should be around 2ft/60cms high and should form a deep valley between the rows.

The other important job is irrigation of the crop but wait until around flowering time before starting regular watering. Remove the flowers to allow the plants to use all of their energy swelling the crop. Sometimes if a flower is missed a small green tomato like plant will grow. These are poisonous and should be removed especially if children are allowed in the area.

How to Harvest

Choose a sunny dry day to lift potatoes. It is better because dry soil is light and this makes the actual digging easier but more importantly the potato skins are clean and free of soil coming out of the ground.  Cut all of the haulms down first and put them on the compost heap or send them of to the local authority’s recycling centre. Use a garden fork to lift the potatoes pushing it into the soil at the base of the earthing up ridge, not down into the soil directly alongside the remains of the haulm. Gently lift, taking care not to strain your back with a shaking action to get rid of the soil and leave the harvested potatoes on the surface of the soil for a couple of hours for the skins to dry and harden. If it is a particularly sunny day you should cover the potatoes with hessian sacking or several layers of newspaper to prevent them turning green.  Always re-dig the area to seek out any “volunteers” that may have been left in the soil. Only lift a quantity of potatoes that you know that you can bag up and put into store by the end of the session,. Don’t leave any lying about for another time.

Storing

Potatoes have to be stored in darkness to prevent them turning green. Don’t use plastic bags or sacks these will make the potatoes sweat and go rotten. Use double thickness paper potato sacks they will keep out the light and provide some insulation against frost. Inspect every potato tuber before putting it into store only select clean, firm; healthy tubers use the dodgy ones first.  Store the sacks somewhere that is vermin free, dry not heated and frost free. Open up the sacks and inspect the potatoes regularly just in case a rotter slipped through at inspection time. When potatoes begin to rot they smell foul and quickly turn to mush.

Issues

Potatoes are vulnerable to quite few diseases the most devastating of them is potato blight sometimes called late potato blight because it appears towards the end of summer (see our information sheet).

Blackleg can be more of a problem in the north especially in wetter seasons. It is one of the earliest diseases usually around June. Affected plants have a stunted appearance with pale yellow/green foliage. The leaves appear stiff and erect and are smaller than usual. The main damage is at the base of the stems. They are black and diseased and the seed potato will be completely rotten. Dig up and destroy all affected plants.

Common Scab is more prevalent on lime/alkaline soils. It forms small raised patches on the skin of the tubers. It is a nuisance because the potatoes have to be peeled more deeply to get under the scab affected areas.

The most common pests are aphids. They can introduce and spread virus problems throughout the crop. Spraying regularly with insecticidal soap with control them.