As summer brings warmer weather, houseplants need more water – from being as dry as you dare in winter to always moist through summer. Avoid saturating the pots , however, as overwatering is damaging. If root balls become overly wet, dry them with kitchen towel and put back in their pots.
Feeding every fortnight with a houseplant or seaweed fertiliser will keep plants healthy.
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Only cacti and succulents can tolerate hot, sunny, south-facing window sills, so move other plants to less bright places.
South-facing conservatories can be difficult to manage unless ample shading and ventilation can be arranged. Plants stressed by excess light and heat are vulnerable to red spider mite . If this occurs, place the plants out in the garden if possible, where the cooler, more humid air will be less favourable to the mites.
Lanky plants – cissus and coleus, for example – can be shortened to make them bushier, with the excised shoots used to make cuttings. Where height is wanted – as with taller ficus – extend the cane by lashing a new cane to the old with discreet cable ties.
Cuttings of soft shoots of houseplants root readily in most cases and, if taken now, have all summer to grow into sturdy plants that can resist winter conditions.
Sever the shoots just below a leaf node with a sharp blade such as a trimming knife, to make 8cm to 10cm cuttings. Remove leaves from the lower two-thirds. Insert the cuttings using a dibber or small stick into a pot filled with equal parts peat-free potting compost and perlite or cork granules. Water well and lightly cover with a plastic bag and leave to root in a lightly shaded place. Expect roots in a few weeks, when the cuttings can be potted.
Consider repotting when frequent watering is needed or plants no longer grow and look unhealthy, roots are growing through the drainage hole or the plant is rather large for its pot. Move the plant into a larger, but not very large pot using peat-free houseplant potting compost . General peat-free potting compost can support sciarid or compost gnats, but specific houseplant material is formulated to discourage these nuisances.
Where a larger pot would be inconvenient, removing a third of the roots and potting compost, then replacing the plant back into the same pot with fresh compost is effective.
Repotting in early summer gives plenty of time for the plants to develop strong root systems that will make them less vulnerable to inadvertent overwatering in winter.
Outdoor air is more humid than indoors, so opening windows, as long as there is no strong draught, helps plants as much as people.
Plants benefit from a spell outdoors in summer , where the humidity, summer rain and good light lead to healthier plants. They should be brought back indoors before colder autumn weather.
Outdoor summer light can be excessive for plants acclimated to the less-illuminated indoors, so keep them in shade at first and gradually acclimatise them to brighter conditions. Citrus in particular show an amazing improvement once outdoors.
Once outdoors, houseplants can occasionally be at risk of pests. All problematic organisms are most easily controlled by early action – so inspect plants every few days, looking for holes, slime trails, twisted leaves and honeydew . Special nematodes can be watered on to control slugs, while snails can be picked off. Greenfly and scale insects are best addressed by products that work by physical action such as SB Plant Invigorator.
In late summer, vine weevil can lay their eggs in pots. A pre-emptive September treatment of a nematode product prevents damage.
The RHS is a charity inspiring everyone to grow via its research, advisory, outreach, shows and gardens. For more information, visit: rhs.org.uk
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