Hardening off and transplanting, the facts

There is no operation more critical in the kitchen garden than that of transplanting young seedlings into the open ground. This is because the plants are in full growth and therefore very prone to receiving a check if exposed to cold or drying winds. Yet we are asking them to adjust to such tough conditions AND to root into a cold soil very different to the warm potting compost they have been used to

Even if given every care and the very best conditions most plants will still unavoidably receive a slight check when transplanted, and the more so the older they are. This can be utilised to spread the harvest of say lettuces; one sowing with half transplanted and half left in situ then crops over a longer period with two peaks at least a week or so apart, if each lot is again subdivided and transplanted another time then but the one sowing can be made to crop over several weeks. But there is a danger that the checks some plants receive will be so severe the plants may bolt and prematurely flower.

Indeed simply the prevention of bolting is a major goal when transplanting especially with the more difficult vegetables such as cauliflower, onions and roots. All of these bolt very quickly if they get stressed at all let alone checked. Of course almost all plants will bolt if moved too big, too late, or in dry or windy conditions. Usually with care bolting can be avoided, but to produce the best quality crops we must aim at not just avoiding an obvious check but at making the transition from the shelter of the greenhouse to the open ground as smooth and stress free as possible.

The most important factor is the strength of the roots and not the size of the leafy top growth. A plant with a good strong set of roots will always move the best. Even more so if these roots are not too disturbed in the move, particularly for some such as sweet corn which resent transplanting. Thus most plants are best transplanted from individual pots or cells rather than from torn apart multi-sown clumps. (Though a few such as beet and onions can be planted out as undisturbed cells of three or five seedlings growing together.)

The plants must not be allowed to become root bound though. The biggest problem for most gardeners is that while waiting for outdoors to warm up the plants sitting indoors get bigger and outgrow the accommodation available for them. They also need potting up as any check to the root system early on will seriously reduce yields. Better to keep the plants indoors and pot up again than plant out too soon into cold ground!

Plants are reluctant to push their roots out of their balls of compost where the planting soil is of a very different nature. Thus the soil, and spoil, around each planting hole should be generously, and uniformly, mixed with some compost to make it more like the compound the plants are started off in. In particular if the soil is heavy and badly drained it also helps to work sand into the soil and the bottom of the planting holes to prevent them filling with water!

Likewise it is not a good idea watering the holes immediately beforehand, or worse, just after planting. It is best to prepare and water the holes the day before so the soil is well moistened but not sodden. If you pack wet soil or mud around roots then when the soil dries it cracks and leaves them surrounded by gaps. Soil must be moist not wet then it will pack firmly and grip the roots but not drown them. Roots are not leaves, roots need oxygen so embedding them in mud or under water stops them functioning. Far better that the soil is moist and friable when packed so that minute air spaces are retained to supply the roots with the oxygen they need to re-establish.

It is thus a good idea to water the transplants really well the day before moving so they are plump and can survive longer. But do not over-water as sodden rootballs are more likely to fall to bits than moist ones. If the soil is sufficiently moistened before planting then no watering should be needed for at least a week. Indeed wetting the soil around the plants will just attract slugs!

Slugs, cutworms, birds and other pests are attracted to succulent wilting leaves. New transplants are in greatest danger for the first few days when they are still relatively soft. Once the plants have adjusted to being outdoors they harden up and become less palatable but for at least the first week they need protection. During the same period they are also most at risk from adverse weather so anything that offers protection from pests and weather is doubly advantageous.

I find that individual cloches cut out of plastic lemonade bottles to be the perfect protection for the transplants. Small ones work for the smaller plants such as beet and brassicas, tall ones for sweet corn and the one gallon or five litre bottles are ideal for courgettes, tomatoes and such bigger plants. Of course any protection helps such as surrounding the transplants with fleece or netting windbreaks. Even just a three inch deep plastic ring cut from a lemonade bottle will keep off the wind from small plants and act as a barrier to most pests.

But even with extra protection there is still a need to harden off the plants before putting them into the cold soil. Hardening off is acclimatising the plant to the colder soil and air temperatures giving the plant time to rearrange it's metabolism to cope with the new conditions. When plants grow under glass the lower light and higher temperatures make the plants produce large soft leaves. When they go outside, even under individual cloches, they must have smaller harder leaves. The old leaves need time to stop growing and harden and new ones to form that are more suited. So it is important never to move the plants straight out in one go but always to let them adjust slowly.

Traditionally hardening off is done by putting the plants outdoors for the day and bringing them in for the night for at least three days but preferably for longer. I find that the process is even better if the plants go through several stages beforehand rather than just this last one. And the same applies if you have bought in plants such as tomatoes as these are rarely hardened off by the nursery!

First my plants are moved from the greenhouse staging onto the floor where they are in colder air but they are still in full light. After a couple of days they are moved to a coldframe where they are open to the air in the day but covered at first every night and then later only on cold nights. A week or so in the cold frame and then they can be stood in front of a south facing wall of a shed. After a couple of days there they are then used to the wind as well as the cold and can cope with going into the open ground.

Given such ideal hardening off the plants do not always need the extra protection of individual plastic cloches; however these dramatically improve results if used, particularly in years with erratic weather. It is also possible to put the cloches in situ a week or two ahead of planting and this will then pre-warm the soil giving even better results.

Naturally if the plants are under such cloches then they need equal care when these are removed. Fortunately by that time the weather has settled and late frosts are unlikely so the plants are not so threatened. I remove the cloches on the morning of a dull overcast day so that the plants can harden further before bright sun or strong winds return. For the most succulent plants such as beets I replace the cloches by wire guards to keep the bigger pests away a bit longer.

Of course some crops are easier as they are moved from a seedbed to their final position. Although these do not need hardening off like plants started under cover all the other points about transplanting are pertinent. Additionally plants growing in a seedbed do not have a containerised root system so it is harder to lift them with an intact rootball. I endeavour to sow thinly, thin early and then lift with a trowel most carefully so that a large cohesive rootball is retained.

With the brassicas it is very good practice to move them twice. The first time, when two to four real leaves are formed, they are lifted and replanted in the same place. This breaks the taproot causing the plants to make bushier root systems that transplant better the second and final time. Indeed brassicas with a single root that can be dibbled in are barely worth growing compared to the plants that grow from a well formed bushy root system.

To summarise; transplanting is the most important operation we undertake for many plants. If we take care all will go well and we are almost assured of a good crop, if we are careless the damage may not be observed until too late. In other words baby plants are a bit like human babies, they need unbelievable care while they are small especially when you move them from the warm to the cold outdoors.