Gardening: When to start growing potatoes

Like many plants, there are a number of potato varieties available. Photo / 123RF

The sight of early daffodils popping into bloom is a sign we are heading towards spring. With some fine days again we can start to see that day length is gradually increasing too.

This means it is time to get together the beginnings of the spring vegetable garden. July is a good month to start off growing the first spring and some summer vegetable plants from seed.

This must be done in a warm situation where seed trays can be kept above 12C while they germinate. Inside the house is most ideal because outside the conditions are too cold for most vegetable seeds to germinate.

Depending on the variety, plants would be ready for planting outside in late August or early September. Following on from last week's column on seed sowing we are now looking in depth at potatoes.

July is also the time to start the production for growing potatoes. The often-asked question is; "do I need seed potatoes or can I plant a few potatoes left in the bottom of the pantry that I purchased from the supermarket?" The answer is to purchase "certified seed potatoes".

This means the seed has been assessed by an authority called Potatoes NZ. This certification is to ensure the seed is free from viruses, which can significantly reduce the yield.

It is important to select a good spot for them to grow because potatoes need a sunny (at least five hours a day) position where they will not be competing for moisture. They also benefit from being in a sheltered area that does not receive strong winds.

Next, the soil needs to be well worked – this means it has been dug over to a depth of 25-30cm. This will allow the potatoes to grow and multiply. If the soil is very heavy and clay-like, then creating raised beds to ensure good drainage will be beneficial.

If on the other hand the soil is very sandy, as it often is in parts of Gonville or Castlecliff, then the addition of organic matter such as compost or sheep pellets will be beneficial because they not only add nutrients, but will help hold moisture for longer.

Potatoes are a tuber that form on part of the stem that is underground. By "mounding up" soil around the stem as the plants grows, you will increase the amount of stem underground and the yield from each potato plant.

What are the economics of growing potatoes? As a benchmark for you, each seed potato planted should yield 15-25 potatoes. So a 2kg bag of seed potatoes (containing about 24 potatoes) will yield 360-600 potatoes (45-75kg.) So, if you have space available, give growing potatoes a go!

Like many plants, there are a number of potato varieties available. The way to choose which one to plant is to know when you want them to mature and how you like to eat your potatoes! Have a look in store and check out the varieties. Three favourites are agria, heather and jersey benne.

Agria is an early main cropper, and has a long oval shape with yellow flesh that is perfect for roasting, chips and wedges. Heather is a main-crop potato that is oval and has a distinctive purple skin with white flesh – it's great for boiling, roasting and casseroles. Jersey benne is an early variety with a kidney or oval shape, whose white skin and flesh are excellent for mashing or boiling.

If you do not have enough space in your garden for a row or two of potatoes you can grow them quite successfully in containers, large plastic bags or a bucket (say 35-litre) using a good-quality mix such as Tui Vegetable Mix.

Make sure there are plenty of drainage holes. Place tubers in the potting mix near the bottom of the container covering them about 50mm. As the sprouts grow, keep adding the vegetable mix until it is up to the brim of the container, add a dressing of potato fertiliser each time the vegetable mix is added. Harvest when the tubers are ready.

Here is an easy seven-step process for delicious home-grown spuds. Ensure you select varieties that suit the way you like to eat your potatoes because some varieties will roast well, while others are excellent in potato salads.

Step 1
Site selection; taking into account the factors previously discussed - sun, moisture and wind.

Step 2
Purchase certified seed potatoes, spread out in a warm light location and leave to grow sprouts (about three weeks.)

Step 3
Prepare the soil for planting. Remove any weeds from the area and dig over the soil to 25-30cm deep, working the soil to a fine texture. Spread both potato fertiliser and garden lime over the soil and mix well so that none will come into direct contact with the potato tubers when they are planted.

Add gypsum to the garden if the soil is clay and raise the bed to improve drainage if required. If the soil is very light then the addition of compost or sheep pellets will improve moisture retention.

Step 4
Plant seed potatoes about 10cm deep, 35-45cm apart and in rows about 75cm apart. This gives a good amount of space to later mound up soil around each plant.

Step 5
Check weekly. When shoots have emerged and are reaching 20cm high it is time to start mounding. Mound up the soil, almost completely covering plants, continue to check regularly.

Once they have grown another 20cm mound them up again. Having so much of the stem covered will ensure a decent crop as tubers form up the buried stems. Sprinkle the soil around the plants with potato fertiliser each time before you mound the soil up.

By covering the stems of the plants with soil you are also providing protection from any late frosts. Any tops remaining should be covered with frost cloth if a frost looks likely.

Step 6
Pest control and care. The potato/tomato psyllid has become a problem in this area. The symptoms often don't show up until it is too late because the insects can be seen only under a microscope.

However, it is easily controlled using Yates Mavrik or Yates Success Ultra on crops growing from November to April.

If you like to avoid using sprays then I suggest planting sooner than later, because the insect problem becomes more prevalent as summer progresses (December onwards). By starting your potato crop now they will probably be ready for harvest before needing treatment for psyllid.

Step 7
Harvest. You know your potatoes are ready for harvest when the plants flower. There are a few varieties (such as rocket) that do not flower, so in this situation it can be useful to mark on the calendar when they would be due.

Generally, early varieties will be ready to harvest about three months from planting while four months for late and main-crop varieties is more suitable. The later and main-crop varieties are usually better for storing.

Gareth Carter is general manager at Springvale Garden Centre.